Metal oxide semiconducting interfacial layers for photovoltaic and photocatalytic applications

Naveen Elumalai; Chellappan Vijila; Rajan Jose; Ashraf Uddin; Seeram Ramakrishna

doi:10.1007/s40243-015-0054-9

Metal oxide semiconducting interfacial layers for photovoltaic and photocatalytic applications

Elumalai, Naveen; Vijila, Chellappan; Jose, Rajan; Uddin, Ashraf; Ramakrishna, Seeram 2015-07-03 00:00:00 Mater Renew Sustain Energy (2015) 4:11 DOI 10.1007/s40243-015-0054-9 REVIEW PAPER Metal oxide semiconducting interfacial layers for photovoltaic and photocatalytic applications 1 2 3 1 • • • • Naveen Kumar Elumalai Chellappan Vijila Rajan Jose Ashraf Uddin Seeram Ramakrishna Received: 6 November 2014 / Accepted: 7 April 2015 / Published online: 3 July 2015 The Author(s) 2015. This article is published with open access at Springerlink.com Abstract The present review rationalizes the signiﬁcance Keywords Photovoltaics Metal oxide semiconductor of the metal oxide semiconductor (MOS) interfaces in the Interfacial layers Renewable energy Photocatalysts ﬁeld of photovoltaics and photocatalysis. This perspective considers the role of interface science in energy harvesting using organic photovoltaics (OPVs) and dye-sensitized Introduction solar cells (DSSCs). These interfaces include large surface area junctions between photoelectrodes and dyes, the Growing economies and increasing population demand interlayer grain boundaries within the photoanodes, and the more energy in the coming years. The prospect of global interfaces between photoactive layers and the top and warming and limited fossil fuel reserves necessitates radi- bottom contacts. Controlling the collection and minimizing cal changes in our global energy production and con- the trapping of charge carriers at these boundaries is crucial sumption patterns. Primary supply of sustainable and eco- to overall power conversion efﬁciency of solar cells. friendly energy is one of the major challenges of the Similarly, MOS photocatalysts exhibit strong variations in twenty-ﬁrst century. A recent report on 2030 Energy Out- their photocatalytic activities as a function of band struc- look by BP points that an additional 1.3 billion people will ture and surface states. Here, the MOS interface plays a grow into new energy consumers by 2030 [1]. Exxon’s vital role in the generation of OH radicals, which forms the 2040 Energy Outlook projects 85 % growth in global basis of the photocatalytic processes. The physical chem- electricity demand during 2010–2040 [2]. Emerging non- istry and materials science of these MOS interfaces and OECD countries alone will experience a 150 % surge in their inﬂuence on device performance are also discussed. electricity demand. However, to have such quanta of energy, the current energy growth of 1.6 % per year would require at least 35 years; therefore, a crisis is inevitable. This increased energy demand is one part of the story; the & Naveen Kumar Elumalai other part is depleting natural resources, increased pro- n.elumalai@unsw.edu.au duction cost and high environmental concerns such as Chellappan Vijila global warming due to excessive use of fossil fuels. Over c-vijila@imre.a-star.edu.sg 85 % of the primary supply in the present-day energy mix is contributed by fossil fuels, thereby putting the life sus- School of Photovoltaic and Renewable Energy Engineering, tenance at an increased risk in the planet [3]. To point out a University of New South Wales, UNSW, Sydney, NSW 2052, Australia consequence of increased energy production cost, many gas wells generate 80–95 % less gas after just 3 years Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), contrary to the predictions that their lifetime would be 3 Research Link, Singapore 117602, Singapore 40 years. Given this swift weakening in natural gas pro- Universiti Malaysia Pahang, 26300 Pahang, Malaysia duction from newly drilled wells, it would be essential to drill 7200 wells per year at a cost of 42 billion dollars, to National University of Singapore, Singapore 117576, sustain the present level of natural gas production [4]. All Singapore 123 11 Page 2 of 25 Mater Renew Sustain Energy (2015) 4:11 these concerns focus our attention towards clean, sustain- other hand, the electrochemical properties of the MOS such able, and zero cost sources of energy; i.e., the solar energy as band edge energies determine their success as and its conversion into electrical energy as the most con- photocatalysts. venient for use in modern life. A total of 36,000 TW solar This review emphasizes on the application of MOSs as energy strikes the Earth. Assuming an efﬁciency of 25 %, a electrodes and interfacial layers in the technologies that solar cell farm of area *367 km 9 367 km in the Sahara involve photon-assisted physiochemical processes. In desert would meet the projected energy demand. For Sect. 2, the role of MOSs in photovoltaics—dye-sensitized comparison, this area is only 0.3 % of 9.4 million km of solar cells (DSSCs) and organic solar cells (OSCs)—are the Sahara desert. Therefore, the sun could be a single discussed. The interfacial processes and energetics solution to all our future energy needs [5–7]. involved in the charge injection and extraction properties Similarly, growing population has also increased the use of these materials are elaborated. In Sect. 3, the effect of of dyes in many industries such as textile, furniture, MOSs in photocatalytic systems is addressed focusing on chemical and paint. The dumping or accidental discharge dye degradation. Recent advances in photocatalysis of dye wastewater has triggered a substantial amount of involving plasmon/metal oxide interface is also discussed. environmental and health problems, urging researchers globally to develop universal methods to treat dye wastewater efﬁciently. Until now, orthodox methods such Photovoltaics as coagulation, microbial degradation, biosorption, incin- eration, absorption on activated carbon, ﬁltration and sed- Recent advances in solar energy conversion technologies imentation have been employed to treat dye wastewater [8, based on organic semiconductors as light-harvesting layer, 9]. Recently, an approach called advanced oxidation pro- such as dye-sensitized solar cells and organic solar cells, cess was developed to treat dye wastewater treatment [10]. employ MOS nanostructures for efﬁcient charge extraction This method involves the process of generating strong OH and transportation between the electrodes and organic radicals for breaking down the complex molecules. Solar molecules. The transition MOSs are well known for their energy serves as the cheapest and efﬁcient source of energy ability to exchange charges with condensed molecules, for generating these radicals through the process of pho- making them a viable and cost-effective candidate to be tocatalysis. Thus, proper engineering and optimization of used in the photovoltaic devices. the materials can provide viable and efﬁcient ways to Solar cells are classiﬁed into different schemes based harness this abundant resource for photovoltaic and pho- either on the historical evolution or on their principles of tocatalytic applications. operation. The class of solar cells based on a p–n junction Semiconductors, deﬁned as materials with band gap is the ﬁrst of its evolution and, therefore, are typically energy B5 eV, are essential to absorb solar energy for called ﬁrst-generation solar cells [11, 12]. Semiconductors, enabling the above tasks; a multibillion dollar semicon- either elemental such as Si or Ge or compounds such as ductor industry is in operation with diverse types of GaAs or InP, are materials of choice to build p–n junction semiconductors including elemental and compound semi- solar cells. Photoexcited carriers in the p–n junction are conductors. Among them, metal oxide semiconductors separated into mobile carriers by the built-in-electric ﬁeld, (MOSs) represent a domain of relatively inexpensive and or band bending, at the junction between the p- and the environmentally benign class of materials with a diverse n-type semiconductors [12–14]. The photovoltage in the p– range of properties owing to their rich structural diversity. n junction is the difference in quasi-Fermi levels (i.e., the Both natural and synthetic MOSs have diverse applica- band bending) of n-type and p-type regions. A typical tions. The properties of MOS can be tailored in many ways, device consists of a 5 lm-long n-type semiconductor and viz., varied choice of morphologies, introducing oxygen 300 lm-long p-type ones, i.e., the minority carriers in the vacancies, doping, and so on. p–n junction are expected to travel *300 lm for efﬁcient In photovoltaics, MOSs serve as a scaffold layer for charge collection, which requires rigorous control on their loading dyes in dye-sensitized solar cells (DSSCs) and chemical purity [12, 15]. Requirement of such extreme organic–inorganic hybrid perovskites in perovskite solar purity of the semiconductors is one of the major cost cells (PSCs), as well as electron and hole transport layers in limiters of the ﬁrst-generation solar cells. The p–n junc- DSSCs and organic solar cells (OSCs). The function of tions are typically built on single crystalline and poly- scaffold in DSSCs is to facilitate charge separation and crystalline platforms. The latter polycrystalline overcomes charge transport, whereas that of the transport layers is to the cost limitations on chemical purity, however, at the conduct one type of charge carrier block to the other type. expense of the photovoltaic conversion efﬁciency (PCE) Therefore, tailoring their properties is inevitable to develop [14, 16]. Whether or not a p–n junction is made from single high-performing photovoltaic devices using them. On the or polycrystals, inherent limitations between the absorption 123 Mater Renew Sustain Energy (2015) 4:11 Page 3 of 25 11 and electron emission in those crystals put a theoretical metals and are often characterized as a mobile excited limit on the photovoltaic conversion efﬁciency in p–n state. An exciton can be considered as a quasi-particle with junction solar cells, known as a Schokely–Queisser limit. an electron in the conduction band (or lowest unoccupied The Schokely–Queisser limit predicts a theoretical upper molecular orbital, LUMO) and a hole in the valence band limit of 32 % PCE for single-junction (p–n) solar cells (or highest occupied molecular orbital, HOMO). When a [12]. semiconductor (molecule, crystals, or clusters) is anchored The second-generation solar cells are based on the with another material whose conduction band (LUMO) lies charge separation at an interface either between two con- at lower energy, then the exciton dissociates into mobile jugated polymers or a ﬂuorophore molecule conjugated carriers (or free carriers) at the interface of the material with a metal oxide semiconductor. The National Renew- system (Fig. 2). This process is the basis of ESCs. Exam- able Energy Laboratory (NREL) at Colorado, USA, cate- ples of this type of ESCs include organic solar cells gorize them as ‘‘emerging solar cells’’ (Fig. 1). In the third (OSCs), dye-sensitized solar cells (DSSCs), and quantum generation, semiconducting nanocrystals of size in the dot solar cells. Conjugated polymers and/or organic quantum conﬁnement region, known as quantum dots, is materials such as PCBM, P3HT, etc., are the materials of used as the light harvester [17]. The quantum dot offers the choice in OSCs. In the DSSCs, a wide-band-gap MOS, possibility of many photoelectrons per single absorbed such as TiO , is anchored to a dye. In the third-generation photon of sufﬁcient energy, thereby uplifting the theoreti- quantum dot solar cells, quantum dots are used as light cal conversion efﬁciency over 60 %. The ﬁrst- and second- harvesters [18–20]. generation solar cells are collectively called ‘excitonic Although an upcoming energy technology, photo- solar cells (ESCs). voltaics—the science and technology of solar cells—has In the ESCs, light absorption results in the generation of steadily progressed. One may note that the p–n junction a transiently localized excited state, known as exciton— solar cells made from single crystals have reached a stage usually, a Frenkel type is formed. These Frenkel excitons of performing with theoretical conversion efﬁciency. On cannot thermally dissociate into free carriers in the material the other hand, performance of ‘‘emerging solar cells’’ in in which it was formed. Moreover, excitons are the char- the second and third generation is relatively inferior. acteristics of semiconductor analogs to Fermi ﬂuids in Compared to the ﬁrst-generation solar cells, these emerging Fig. 1 Best research-cell efﬁciencies of different types of solar cells. This plot is courtesy of the National Renewable Energy Laboratory, Golden, CO. (2015) 123 11 Page 4 of 25 Mater Renew Sustain Energy (2015) 4:11 Fig. 2 Schematic illustration of an excitonic solar cell solar cells offer ease of fabrication, ﬂexibility, lower cost, and higher performance efﬁciency. Although p–n junction solar cells have the advantage of high PCE ([20 %) and long lifetime (25 years), ESCs have improved enormously in the past few years demonstrating PCEs as high as 12 % and lifetimes of 10 000 h [21–23]. Progress in photovoltaics has become indispensable to improve the performance and cost-effectiveness of har- nessing the solar energy. As a function of this driving factor, intense research works are carried out across various components of a photovoltaic device. Among them, engi- neering the nanometer (electron) and micrometer (photon) scale interfaces among the crystalline domains is impera- tive for efﬁcient charge transport, as these domains con- stitute the interfacial layers in the solar cells. The interfacial layers include the following: (1) inter-percolated high surface area junctions between photoelectron donors and acceptors, (2) the interlayer grain boundaries existing within the absorber material, and (3) the interfaces between Fig. 3 Existing challenges in the ﬁeld of excitonic solar cells the top and bottom contacts in conjunction with the pho- toactive layers. Optimizing the charge carrier transport discuss the physical chemistry and materials science of across these interfaces is pivotal for the efﬁciency and these MOS interfaces and their impact on device lifetime of the photovoltaic device [24]. The existing performance. challenges in the ﬁeld of excitonic solar cells is represented in Fig. 3. Organic solar cells In this section, the application of MOSs in two main groups of solar technologies that are approaching or have Organic solar cells (OSCs) have attracted immense exceeded 10 % solar power conversion efﬁciency are dis- research interest in the past decade owing to easy fabri- cussed, i.e., dye-sensitized solar cells (DSSCs) and organic cation and low cost [25]. The quest for improvement in solar cells or organic photovoltaics (OPVs). Here, we also 123 Mater Renew Sustain Energy (2015) 4:11 Page 5 of 25 11 power conversion efﬁciency (PCE) of OSCs has been fullerene derivatives along with the electron afﬁnities and actuated by the development of novel photoactive materials ionization energies of high or low-work-function MOS and device architectures [26–28]. Numerous donor–ac- employed in OSCs [42]. These MOSs provide the basis for ceptor (D–A) polymers have been synthesized by varying developing highly efﬁcient and stable ohmic contacts by the highest occupied molecular orbital (HOMO) and lowest means of energy-level bending, vacuum-level shifting and unoccupied molecular orbital (LUMO) levels of the donor Fermi-level pinning at the polymer–electrode interfaces. and the acceptor materials [27, 29–31]. The enhancement Furthermore, the use of oxide interlayers evades the direct in short circuit currents (J ) and open circuit voltages contact between the photoactive layer and electrodes, SC (V ) achieved by means of band gap and interfacial where high densities of carrier traps or adverse interface OC engineering resulted in high PCE *10 %. [25] Further- dipoles hamper efﬁcient charge collection. Moreover, MOS more, development of novel printing and coating processes interfacial layers play a dominant role for developing have been developed leading to roll-to-roll processing of ohmic contacts to maximize the V , because lower built- OC organic solar cells [32–34]. During this course of devel- in potential eventually leads to an increase in dark current opment, transition metal oxide semiconductors (MOS) as well as carrier recombination. have gained profound attention in OSCs owing to their ability to transport/extract charge carriers efﬁciently and Metal oxide semiconductors for OSCs solution processibility which is well suited for roll-to-roll high-volume production. Metal oxide semiconductors (MOSs) for the OSCs can be Organic solar cells (OSCs) mostly employ bulk hetero- p-type and n-type materials, contingent on the position of junction (BHJ) device structure in which the photoactive the conduction band and valence band. For an n-type blend comprises an electron-donating polymer and an material, electron transfer from the LUMO of the acceptor electron-accepting fullerene derivative in a pattern of to the conduction band (CB) of the MOS is the requisite. nanoscaled interpenetrating networks. Fabrication of ohmic For a p-type contact material, the valence band (VB) of the contacts is very important in OSCs, but it is not as MOS is required to match the HOMO of the polymer. The straightforward as that in inorganic solar cells. OSCs wide band gap of the interface materials also serves as a require speciﬁc materials for this purpose with appropriate barrier for carriers of the other sort, thus improving the interface engineering. Poor ohmic contacts with transparent carrier selectivity of the contacts [40, 43–47]. conducting oxides (TCO) or metallic electrodes arise due The main roles of interface materials are: to the (i) misalignment of energy levels or mismatched 1. To align/adjust the energetic barrier height between work function [35, 36], (ii) the formation of interfacial the photoactive layer and the adjacent electrodes. dipoles [37, 38] and (iii) interfacial trap states [39]. Various 2. To materialize a selective contact for carriers of one charge-extracting interlayers have been employed between sort (either holes or electrons). the active layer and the electrodes to develop good and 3. To control the polarity of the device (to make normal efﬁcient ohmic contacts. Among the various interfacial or inverted device structure) as shown in Fig. 5. materials used, transition metal oxide semiconductors 4. To prohibit a physical or chemical reaction between (MOS) are considered as potential candidates owing to the polymer and electrode. their high environmental stability, superior optical trans- 5. To serve as an optical spacer. parency and facile synthesis routes. Open circuit voltage V of OSCs is determined by the OC Cathode interfacial layers for OSCs energy-level alignment between the donor and the acceptor in devices with ohmic contacts; if not, it is determined by the work function difference of the contact electrodes [26, Based on the device architecture (i.e., normal or inverted) 40]. On the other hand, short circuit current (J )is as shown in Fig. 5, the cathode interfacial layer lies in SC determined by the amount of photogenerated carriers pro- conjunction with the low-work-function metal (top elec- duced in the active layer upon illumination and the charge trode) or at the bottom adjoining the TCO electrode. Ini- separation efﬁciency across the photoactive layer. Fill tially, alkali metals or related compounds are used to make factor is determined by the factors such as series resistance, ohmic contacts to the electron acceptors in the bulk shunt resistance, and charge recombination/extraction rate heterojunction/photoactive layer. Cesium carbonate (Cs 2- in the device. Finally, the performance efﬁciency (PCE) is CO )[48] and lithium ﬂuoride (LiF) were used for this determined by the product of Voc, Jsc, and ﬁll factor; purpose owing to their low work function (\3.0 eV). They exhibited good electron injection properties and enhanced which is then normalized to the incident light intensity usually 1 sun at AM 1.5 G [41]. Figure 4 shows the HOMO the V of the device effectively [49]. However, degra- OC and LUMO levels of various donor polymers and acceptor dation issues such as oxidation of alkali metal compounds 123 11 Page 6 of 25 Mater Renew Sustain Energy (2015) 4:11 Fig. 4 Schematic view of the energy levels of metal oxides and orbital energies of some of the organic components used in OSCs. Reprinted (adapted) with permission from ref [47]. Copyright (2011) American Chemical Society Such characteristics make the n-type semiconducting oxi- des an effective alternative to low-work-function metals used as cathode contacts. The proﬁciency of these oxides is demonstrated in both conventional and inverted device geometries [51–56]. ZnO is presently the most widely used electron-trans- porting layer (ETL) in OSCs. Its Fermi level aligns well with the LUMO of electron acceptors [57]. Moreover, it also acts as an effective hole blocker owing to its high ionization potential, thereby increasing the shunt resistance of the device [55, 58]. ZnO is transparent to visible light and absorbs in the UV spectrum, serving as a low band ﬁlter for the photoactive layer. To fabricate OSCs with inverted device geometry, ZnO nanoparticles or colloids are spin coated onto TCO substrates. Solution-processed ZnO nanostructures are obtained from precursor solutions containing Zinc salts through methods such as sol–gel [59], solvothermal [60], or a hydrothermal process [61]. More- over, ZnO NPs are readily synthesized from zinc acetate dehydrate and used as ETL [62–64]. Using such an approach, inverted poly (3-hexylthiophene) (P3HT) cells with a PCE of 4 % have been demonstrated [65]. The Fig. 5 Schematic representation of a normal and b inverted device synthesized ZnO nanostructures have a lot of defect states structure of OSCs [98]. Reproduced by permission of the PCCP Owner Societies in them. The small diameter of the ZnO NPs possesses a large fraction of dangling bonds, giving rise to high density over a period of time leads to poor stability [50]. Therefore, of gap states. Thus, elimination of localized energy states the use of transition MOS such as ZnO and TiO with work in the band gap of the charge transport layers in inverted functions corresponding to the LUMO levels of fullerenes organic solar cells signiﬁcantly augments the operational is considered to be an effective alternative. These MOSs stability of the device in addition to enhanced photovoltaic are well known for their chemical resistance to oxygen and parameters. Therefore, UV exposure is required to improve moisture, optical transparency and solution processibility. the conductivity of ZnO NPs by means of photodoping and 123 Mater Renew Sustain Energy (2015) 4:11 Page 7 of 25 11 defect ﬁlling. UV-ozone (UVO) treatment was also found evaluated and the one with a higher crystallinity was to passivate the surface defect states [57]. With this characterized by a trap depth lower by a factor of two, approach, PDTG-TPD:PC BM-based devices with ZnO when compared with the device annealed at lower tem- NPs as ETL exhibited enhanced device performance with perature [67]. PCE *8% [66]. However, this UV exposure method is Under the testing protocol ISOS-L-1, the devices A not sufﬁcient for optimum device performance. Another (annealed at 240 C) and B (annealed at 160 C) retained approach to achieve optimum device performance was by 64 and 48 % of their original efﬁciency after 400 h of using in P3HT:PCBM-based devices by increasing the ZnO constant illumination, respectively (refer Fig. 7)[67]. nanostructure crystallinity and annealing temperature. Moreover, ZnO nanowires were planted across the ZnO Altering the crystallinity by annealing the ZnO at two nanoparticles to increase the electron lifetime, decrease temperatures, viz. 160 and 240 C, produced a difference recombination, and improve charge collection at the cor- in the density of localized energy states in the band gap of responding electrodes. Reports have shown that incorpo- ZnO. The trap depth in the device annealed at 240 Cis rating electrospun ZnO nanowires could effectively lesser when compared with that annealed at 160 C, as increase the carrier lifetime by twofold [54]. shown in Fig. 6 [67]. The devices fabricated using the ZnO ZnO is also used to form the interconnecting unit in nanostructures annealed at 240 C showed remarkably tandem OSCs. Such an interconnecting unit, basically a higher power conversion efﬁciency (PCE) and IPCE val- tunneling p–n junction, is sandwiched between the two ues. The depth of electronic traps in the two devices was BHJ cell stacks, thereby forming a hole–electron recom- bination zone [68]. In this zone, the Fermi levels of the HTL and the ETL are well matched to minimize the V OC loss in a tandem cell. Thus, ZnO is widely applied in OSCs for research and commercial purposes [68]. Similarly, TiO is also an n-type and wide gap semi- conductor (E * 3.2 eV) with its conduction band minima composed of the Ti 3d band and its valence band maxima composed of the O 2p states [69–73]. In OSCs, TiO ﬁlms are usually processed by sol–gel process; therefore they are in amorphous phase rather than crystalline. The solution precursor is usually prepared using titanium isopropoxide along with solvent additives. TiO ﬁlms are typically deposited by spin coating at optimal speed followed by annealing up to *150–200 C, similar to the synthesis of Fig. 7 Stability of the inverted devices with ZnO (ETL) and MoO (ETL) under constant illumination conditions tested under Protocol Fig. 6 Trap depth (D) in devices A (160 C) and B (240 C) ISOS-L-1. a ZnO with low degree of localized states b ZnO interlayer calculated from ln J vs 1/T curves. Inset shows the variation of trap with high degree of localized states or larger trap depth [57]. depth (D) as a function of U [67]. Reproduced by permission of the Reproduced by permission of the PCCP Owner Societies PCCP Owner Societies 123 11 Page 8 of 25 Mater Renew Sustain Energy (2015) 4:11 sol–gel ZnO discussed in the previous section. OSCs fab- well as the resistance at the photoactive layer/anode ricated with sol–gel TiO ﬁlms have been demonstrated in interface [85]. Besides, MoO HTL also serves as an x 3 both conventional and inverted cells. In conventional cells, optical spacer for improving light absorption, thereby the incorporation of TiO as an ETL exhibited enhanced enhancing the photocurrent [86–88]. Molybdenum oxide is J and FF when compared with devices using aluminum widely used as hole injection material and was considered SC electrodes. OSCs fabricated with solution-processed TiOx as a p-type semiconductor initially. Until recently, UPS ﬁlms serving as ETL and PCDTBT:PC BM blend as studies have exposed that it is of n-type semiconductor and photoactive layer, results in PCE as high as 6 % [74]. the charge transport occurs via the Fermi level being pin- The improvement of PCE was attributed to the improved ned with the valence band of the polymers. Moreover, electrical coupling with PC BM and the improved light vacuum-deposited/thermally evaporated MoO gives the 71 3 harvesting [75]. Here, TiO acts as an optical spacer pro- advantage of precise thickness control in the nanoscale viding more absorption cross section at the photoactive range of about 1–2 nm. Reports have revealed that the layer. TiO was also found to exhibit strong dependence on oxygen deﬁciency in e-MoO results in defect states in 2 3 UV illumination [76, 77]. The UV-activated TiO ﬁlms are energy bands, which raises the Fermi level closer to the shown to degrade much rapidly under continuous illumi- conduction band [89, 90]. nation during regular operating conditions. The UV light However, MoO is highly sensitive to oxygen and causes photodoping of TiO where oxygen is chemisorbed moisture; even the trace amounts of oxygen in the nitrogen- at those sites, leading to unfavorable band bending in the ﬁlled glove box during device fabrication are shown to TiO and thereby hindering charge extraction [78]. UV have detrimental effects on its electronic levels, imposing photodoping is one of the prime issues to be solved to use it severe shortcomings in the device stability [91]. Upon in OSCs as the ETL layer efﬁciently. exposure to ambient conditions (oxygen or air), the work function decreases to 5.3–5.7 eV, which is still adequate Metal oxide semiconductors (MOS) as anode enough to form good ohmic contacts with organic hole interfacial layers transporting materials. Reports have indicated that further reduction of the suboxide results in the growth of gap states The primary requirement of an efﬁcient MOS serving as that would ﬁnally reach the Fermi energy, resulting in the HTL or anode interfacial layer is that its work function metallic behavior of MoO [80, 92]. needs to be high as well as align with the HOMO level of Generally, the hole transport in nanostructured MoO the photoactive polymers [40, 46, 79]. The conjugated layer occurs via the shallow defect states present in its band polymers with high ionization potential cannot form ohmic gap formed as a result of oxygen vacancies [93–95]. These contacts directly in conjunction with the metal electrodes, oxygen vacancies serve as n-type dopants and lead to because of the electron transfer from the metal to the Fermi-level pinning at the photoactive layer–MoO inter- organic photoactive layer. This electron transfer creates a face [96]. The mechanism of the charge transfer process dipole at the interface, leading to reduction in built-in across the MoO interlayer is represented in Fig. 9, which potential of the device. The drop in built-in potential shows that the holes are extracted by injecting electrons increases the series resistance and charge extraction ﬁeld, into the HOMO of the donor (P3HT). Subsequently, the thereby hindering the device performance. The energy holes transferred to MoO hop to the Ag electrode through level interaction at the MOS–organic interface is repre- the shallow defect states generated by the oxygen vacan- sented in Fig. 8 [80]. cies [97]. Numerous vacuum-deposited transition MOS with high Using MoO to replace PEDOT:PSS as the anode work functions, eminent optical transparency, and good interfacial layer (HTL), the resulting devices show com- stability have attracted signiﬁcant research interest. OSCs parable initial performance with much enhanced stability incorporated with these MOSs have demonstrated good as shown in Fig. 10, demonstrating that high-work-func- device performance. Most transition MOSs such as tion n-type MOS can effectively replace PEDOT:PSS in molybdenum oxide, tungsten oxide, and vanadium oxide both conventional and inverted devices [98]. are commonly used in n-type semiconductors for HTL. Reports have shown that a high degree of oxygen These MOSs enable effective Fermi-level pinning and also defects were introduced in the hole-conducting MoO layer -5 increase the built-in potential of the device. The role of by annealing the devices under vacuum (*10 mbar) at these MOSs as HTL is discussed in the following section. nominal temperature (120 C) and time (10 min). The Molybdenum oxide (MoO ) has gained signiﬁcant devices thus fabricated exhibited much higher operational attention for improving device performance and stability in stability, when tested following the ISOS-D-1 (shelf) pro- OSCs [81–84]. The MoO HTL reduces the charge tocol, than control devices annealed conventionally, i.e., in recombination by suppressing the exciton quenching as nitrogen atmosphere. 123 Mater Renew Sustain Energy (2015) 4:11 Page 9 of 25 11 Fig. 8 Energy level at oxide/organic interface. Reprinted with permission from Ref. [80]. Copyright 2012 Nature Publishing Group Fig. 9 Schematic showing the mechanism of hole transport across the molybdenum trioxide (MoO ) interlayer [97]. Reproduced by permission of the PCCP Owner Societies For large-scale production or R2R processing, it is lower than that of the devices made with e-MoO .An necessary to deposit MoO by a solution-processing tech- investigation of better solution-processing conditions and nique instead of vacuum deposition. Reports have shown methods is needed to make it commercially successful [45, that P3HT:PCBM cells with solution-processed MoO 99, 100]. showed a PCE of 3.1 %. Despite the low-temperature Another n-type MOS with high work function is tung- process and acceptable PCE values, aggregation of sten oxide, WO ; similar to MoO , its electronic structure 3 3 s-MoOx is one of the major issues hindering its application is highly determined by its stoichiometry, its crystalline in large-scale processing. However, the work function structure, and processing/deposition conditions. Evapo- values of s-MoOx ﬁlms tend to be lower than that of rated ﬁlms of amorphous WO are generally deﬁcient in e-MoO , thus affecting the quality of the resulting ohmic oxygen, which gives rise to the gap states and n-type contacts. P3HT:PCBM-based devices are made with semiconductivity. Thermally evaporated ﬁlms of WO are s-MoOx, as HTL exhibits a V of 0.55 V which is 50 mV typically oxygen deﬁcient, thereby possessing a large OC 123 11 Page 10 of 25 Mater Renew Sustain Energy (2015) 4:11 105]. The inverted OSC devices fabricated with PCDTBT in the active layer along with TiO as the electron transport layer (ETL) and MoO as the hole transfer layer (HTL) exhibits high Voc of about 91 % with a PCE of 7.2 % as shown in Table 1, owing to low band gap of the polymer and efﬁcient charge transport across the interface [98]. The devices that employ tungsten oxide (WO ) as HTL shows superior performance than vanadium pentoxide (V O )as 2 5 HTL within the same P3HT-based active layer. The V of OC the WO -based device shows signiﬁcant improvement compared to that of the latter owing to reduced charge transport barrier at the interface and lower series resistance [106, 107]. Table 1 shows the photovoltaic parameters Fig. 10 Comparison between the normalized PCEs as a function of obtained for the inverted organic solar cells employing time for PCDTBT: PC BM devices employing PEDOT:PSS and different photoactive layer and interfacial layers. MoO as hole-transporting interfacial layers (HTL). Copyright 2011 Wiley-VCH [98] Dye-sensitized solar cells (DSSCs) amount of gap states. The oxygen deﬁciency also con- O’Regan and Graetzel reported on the dye-sensitized solar tributes to the n-type semiconductivity of WO ; therefore, cell (DSSC) established on the mechanism of novel its electrical properties like work function are also sensitive regenerative photoelectrochemical processes with an efﬁ- to oxygen exposure. The optical band gap of WO of ciency of *7.9 % [73]. Following its success, extensive thermally evaporated ﬁlms is around 3.2–3.4 eV. Oxygen research has been carried out in this ﬁeld to increase the exposure is found to increase further up to 4.7 eV. Its power conversion efﬁciency (PCE) of DSSCs by incorpo- performance as HTL is signiﬁcantly acceptable as PED- rating n-type MOSs such as TiO , ZnO, Nb O , SrTiO , 2 2 5 3 OT:PSS, with the devices exhibiting V of 0.6 V and FF and SnO and their composites as photoelectrode materials. OC 2 of 60 % [44, 101]. Reports have indicated that solution- The wide-band-gap MOSs (Eg [ 3 eV) having suitable processed tungsten oxide with a larger work function band position relative to dye (or photosensitizer) have been -2 increased the efﬁciency to 3.4 % with J * 8.6 mAcm ; used for the fabrication of DSSCs. Owing to the wide band SC V * 0.6 V; FF * 0.6 for a P3HT:PCBM cell. The gap, the MOSs employed for the fabrication of DSSCs OC devices exhibited an enhanced lifetime/stability by main- have absorption at the ultraviolet region. Therefore, pho- taining 90 % of the initial value after being exposed to tosensitizer/dye is responsible for the absorption of light at ambient conditions for nearly 200 h without any encapsu- the visible and near-infrared region. Furthermore, the high lation [102, 103]. These factors make s-WO a viable surface area of nanoporous MOS increases dye loading; candidate than other solution-processed high-work-func- thereby enhancing light absorption leading to improved tion oxides for application in large-scale coating in the R2R performance of DSSCs. In addition to the above-mentioned process. physical characteristics, low cost, natural abundance, and Another n-type semiconducting oxide used as the anode facile synthesis methods of MOS combined with interlayer is vanadium pentoxide, V O . Its band gap is facile solution processibility is another key advantage for 2 5 around 2.8 eV as estimated by UPS and IPES studies [100], the application in DSSCs [71, 112, 113]. revealing that its absorption band partially covers the Among the many wide-band-gap oxide semiconductors absorption band of PC BM. Similar to MoO and WO , (TiO , ZnO, and SnO ) that have been examined as 71 3 3 2 2 the band structure of thermally evaporated V O is highly potential electron acceptors for DSSCs, TiO is the most 2 5 sensitive to the ambient conditions. P3HT:PCBM devices versatile. It delivers the highest efﬁciencies, is chemically -6 fabricated using e-V O (*10 Torr) as an anode inter- stable, non-toxic, and available in large quantities. TiO 2 5 2 layer exhibited an optimum PCE of *3 %. Upon air has many crystalline forms, with anatase, rutile, and exposure, the work function of e-V O further reduces to brookite being the most important ones. The crystal 2 5 5.3 eV along with a signiﬁcant reduction of electron structure of anatase and rutile are based on a tetragonal 4? afﬁnity and increase of defect states. In comparison with symmetry, in which the Ti atoms are sixfold coordinated MoO , research on V O is rudimentary. For a better to oxygen atoms. The main difference between both 3 2 5 understanding of the charge transport mechanism at the structures is the position of the oxygen atoms. In contrast to V O –polymer interface, additional investigations of the rutile, anatase has a smaller average distance between the 2 5 4? interface electronic structures are indispensable [43, 104, Ti atoms; thus, anatase is thermodynamically less stable. 123 Mater Renew Sustain Energy (2015) 4:11 Page 11 of 25 11 Table 1 List of high-efﬁciency OPVs employing MOS interfacial layers Polymer Cathode interlayer Anode interlayer J (mA/cm)V (V) FF (%) PCE (%) Reference SC OC P3HT sol gel-ZnO MoO 10.9 0.57 61.6 3.8 [108] P3HT ZnO NP MoO 12.6 0.63 62 4.9 [109] PCDTBT sol gel-ZnO MoO 10.4 0.88 69 6.3 [110] PCDTBT s-TiO MoO 11.9 0.91 66 7.2 [98] x 3 PDTG-TPD ZnO NP MoO 14.1 0.86 67 8.1 [57] P3HT s- s-TiO WO 7.2 0.59 60 2.6 [111] x 3 P3HT sol gel-ZnO WO 8.19 0.86 67.7 4.8 [106] P3HT sol gel-ZnO V O 10.4 0.56 66 3.8 [107] 2 5 a-PTPTBT sol gel-ZnO VO 11.6 0.82 53 5 [107] P3HT sol gel-ZnO VO 10.1 0.57 67 3.9 [107] The phase transformation from anatase to rutile occurs in connector to the external circuit. The functional principle is the temperature range of 700–1000 C, depending on the similar to photosynthesis: upon photoexcitation, the dye crystallite size and impurities [69, 114, 115]. molecules inject an electron into the conduction band (EC) Rutile has slightly lower indirect band gap (3.0 eV) as of TiO , leaving the dye in its oxidized state (D?, also compared to anatase (3.2 eV), which is attributed to a referred to as dye cation). The dye is restored back to its negative shift of the conduction band in anatase by 0.2 eV. ground state by electron transfer from the redox pair [72]. The bonding within TiO is partly covalent and partly The mechanism of operation of the DSSCs is illustrated in ionic. Therefore, stoichiometric crystals are insulating [71, Fig. 12. 113]. However, a signiﬁcant amount of trap states are The regeneration of the sensitizer by iodide/tri-iodide induced during most synthesis routes, which are due to electrolyte results in the recombination of the conduction oxygen vacancies. These vacancies can also be formed band electron with the oxidized dye. Diffusion of reversibly under reduced pressure and/or elevated temper- e through the nanocrystalline MOS ﬁlm to the substrate 3- ature, which can lead to a variation in conductivity by electrode and diffusion of the oxidized redox species (I several orders of magnitude. The oxygen vacancies cause ions formed by oxidation of I ) through the electrolyte 3? the formation of Ti state, which dope the crystal nega- solution to the counter electrode facilitates both charge tively (n-type). In contrast to other semiconductors of carriers to be transferred to the external circuit, where the similar band gaps (e.g., ZnO), it does not photodegrade energy is transferred to the external load and the regen- upon excitation. On the other hand, TiO is less stable to erative cycle is completed by electron transfer to reduce 3- - UV degradation compared to tin oxide (SnO ), owing to its I to I [20, 71]. It is of critical importance for the high band gap and high work function. However, low functioning of the cell that the injection of electrons into electron mobility (l ) through mesoporous TiO the TiO is many orders of magnitude faster than any n 2 2 2 -1 -1 (*0.1 cm V s ) is a crucial issue and imposes severe recombination (loss) of charge carriers. Moreover, the limitations in enhancing the g of DSSCs closer to the most important recombination process is the direct elec- theoretical limits. The energy levels of the conduction and tron transfer from the conduction band of TiO to the valence bands of the MOS used in DSSCs are shown in redox electrolyte without passing the external circuit [71, Fig. 11. In the standard version of DSSCs, the typical ﬁlm 113]. thickness is 2–15 lm, and the ﬁlms are deposited using Interfacial electron transfer is the process in which the nano-sized particles of 10–30 nm. A double-layer structure excited electron from the LUMO of dye is injected into the can be fabricated, where an underlayer of thickness conduction band of MOS (photoanode) with the rate con- 2–4 lm is ﬁrst deposited using larger (200–300 nm) size stant k [113]. The kinetics of the interfacial electron particles that acts as a light-scattering layer to induce a transfer at the interface strongly relies on the energetics of phototrapping effect [20, 69, 113, 114]. the MOS/dye/electrolyte interface and the density of Dye-sensitized solar cells are based on an MOS nanos- electrons in MOS photoanodes (i.e., Fermi-level of metal tructure that is sensitized with a ruthenium-containing dye oxide). The interfacial electron transfer occurs mostly in a molecule. Different types of MOS photoanodes and their time scale of several picoseconds. Electron injection rate of -12 -1 respective band energies are shown in Fig. 11. A redox [10 s has been reported for several sensitizers and electrolyte and two conducting glass substrates provide the MOS ﬁlms [72, 112, 113]. 123 11 Page 12 of 25 Mater Renew Sustain Energy (2015) 4:11 Fig. 11 Band energies of conduction band (CB) and valence band (VB) of different metal oxides used in DSSCs. Reprinted with permission from ref [72]. Copyright 2001 Nature Publishing Group Injected electron transfers across the mesoporous layer conversion efﬁciency of DSSCs beyond the present record of MOS to the transparent conducting oxide (TCO). The of 11–15 % [69, 114–116]. efﬁcacy of this process is mainly determined by the dif- However, low electron mobility (l ) through meso- 2 -1 -1 fusion coefﬁcient of electrons (D ) and the electron life- porous TiO (*0.1 cm V s ) is a crucial issue and e 2 time (s ). Hence, the nanostructured MOSs with particle imposes severe limitations in enhancing the g of DSSCs size greater than their exciton Bohr radius signiﬁcantly closer to the theoretical limits [117]. One of the main inﬂuence the photoconversion efﬁciency of DSSCs. The hurdles due to inferior l is the electron recombination factors that affect the DSSCs performance are (i) the with the electrolyte if the photoanode layer thickness is mesoporous nature and high surface area of the MOS larger than the diffusion length, the transit length above photoanodes, which allows large amount of dye anchoring which electrons are lost via recombination. resulting in the enhancement of the absorption cross Tin oxide (SnO ) nanostructure, on the other hand, is a section, and (ii) larger amount of density of states (DOS) well-known transparent conducting oxide for nano-elec- 2 -1 -1 in MOS than the molecular orbital of dye enables tronics due to high l (10–125 cm V s ) and wider band speedier injection of electrons from the dye molecule to gap (*3.6 eV) [118–121]. However, its conduction band the MOS. Considering the scenario, inefﬁcient charge minimum occurs at an energy lower than that of TiO [72] transport in the nanostructured MOSs originate from the and, therefore, DSSCs with SnO electrode usually give trapping and detrapping of electrons at the surface atomic low open circuit voltages (V B 600 mV) [122]. OC states in the electronic band. Moreover, the nanostruc- Recently, V up to *600 mV has been achieved by OC tured MOS photoanodes consist of large amount of sur- preparing the SnO core/shell electrodes and/or making face atoms resulting in greater degree of trap density. The composite electrode with other wide-band-gap semicon- trapping and detrapping process lowers the kinetic energy ductors [123–125]. To increase the PCE of SnO of the mobile electrons, eventually leading to poorer cell based DSSCs, several approaches have been consid- performance. Quantiﬁcation of the traps and their subse- ered which includes: (i) modifying the electrode surface quent elimination could improve the photovoltaic [126], (ii) modifying electrolyte composition [126, 127], 123 Mater Renew Sustain Energy (2015) 4:11 Page 13 of 25 11 an electrospinning technique as shown in Fig. 13. The ﬂowers also exhibited an enhanced Fermi energy resulting in higher electron mobility [130]. Furthermore, DSSCs fabricated using the SnO ﬂowers resulted in V 2 OC *700 mV and one of the highest photoelectric conversion efﬁciencies achieved using pure SnO . The study also demonstrated that the ﬂowers are characterized by higher chemical capacitance, higher recombination resistance, and lower transport resistance compared with ﬁbers. The effective electron diffusion coefﬁcient and electron mobility in the ﬂowers were an order of magnitude higher than that for the ﬁbers (Fig. 14). One of the most critical challenges in DSSCS research is the rapid recombination rate between the electrons in the conduction band of TiO and the oxidized redox mediator of the electrolytes. Advances in solid-state DSSCs with spiro-OMeTAD as HTM have shown that its usage is limited by the thickness of the photoanodes (*3 mm) due to incomplete percolation [131]. Therefore, pore ﬁlling has become an area of intense research to improve the hole injection dynamics and reduce the recombination rate with Fig. 12 Schematic of the functional principle of a dye-sensitized thickness [132, 133]. Several approaches have been solar cell. E and E are the position of the valence and conduction VB C investigated to improve the charge collection in liquid- and bands of TiO , respectively. The open circuit voltage V is deﬁned 2 OC solid-state electrolytes by using one-dimensional ZnO and by the difference between the Fermi-level E and the redox potential * ? E of the iodide/iodine couple. D /D are the ground state and D / F,redox TiO nanostructures as photoanodes [134–137]. D is the excited state of the sensitizer from which electron injection 3D photoanodes for DSSCs was also developed to into the TiO conduction band occurs overcome such drawbacks. Recently, a novel bottom-up 3D host–passivation–guest (H–P–G) electrode was developed (iii) combining with other MOS nanoparticles [125], and which enabled complete structural control of the nanos- (iv) by using a core–shell conﬁguration of suitable energy tructure that favors efﬁcient electron extraction and band-matched MOS [125, 128, 129]. recombination dynamics with enhanced optical scattering Flower-shaped nanostructures of an archetypical trans- properties for improved light harvesting. This 3D nanos- parent conducting oxide, SnO , have been synthesized by tructure when employed as photoanode in DSSCs Fig. 13 SEM images of SnO photoanodes: a ﬁbers and b ﬂowers [130]. Reproduced by permission of The Royal Society of Chemistry 123 11 Page 14 of 25 Mater Renew Sustain Energy (2015) 4:11 Fig. 14 Schematic illustration of the fabrication method for a 3D host–passivation–guest dye- sensitized solar cell. Reprinted (adapted) with permission from ref [140]. Copyright (2011) American Chemical Society signiﬁcantly improved photocurrent, ﬁll factor, and most chain reaction involved in the photocatalytic oxidation importantly the photovoltage of the device. [114, 138– processes. This technology is preferably used in photocat- 140]. DSSCs employing novel porphyrin sensitizers with alytic dye degradation owing to advantages such as (1) low cobalt (II/III)-based redox electrolyte exhibit high PCE or no toxins, (2) being cheaper, (3) exhibiting tunable [12 % as shown in Table 2 [141]. The speciﬁc molecular physiochemical properties by modifying the nanoparticle design of porphyrin sensitizers signiﬁcantly retards the rate size and doping concentration and (4) good photocatalytic of interfacial back-electron transfer from the conduction lifetime without undergoing substantial loss over a period band of the nanocrystalline titanium dioxide photoanode to of time. [145]. Generally, metal oxide photocatalysis is the oxidized cobalt mediator, leading to the attainment of carried out via advanced oxidation process (AOP) which is extraordinarily high photovoltage of about 1 volt [141]. performed by employing a strong oxidizing species of OH Other similar DSSCs employing various dyes and redox radicals usually produced in situ. The OH radicals form the shuttle mediators are listed in Table 2. trigger to initiate a sequence of reactions that crumbles the complex dye macromolecule into simpler and smaller, less harmful components [10, 146, 147]. Photocatalysis Photocatalysis is the key process that enables the conver- Basic concept of photocatalysis sion of solar energy into chemical energy needed for the decomposition of dyes or organic pollutants. The photo- Photocatalytic reactions are basically a multi-step process catalytic reactions usually occur on the surface of the involving oxidation and reduction reactions as illustrated in semiconductors. Considering the scenario, metal oxide Fig. 15. The photocatalytic processes comprise three fun- semiconductor (MOS) photocatalysts are employed as damental reaction pathways: (1) Photons are absorbed by activators that assist in catalyzing the complex radical the photocatalysts upon illumination from the light source. Table 2 List of high-efﬁciency DSSCs employing MOS photoanodes Photoanodes Dye Redox couple J (mA/cm)V (V) FF (%) PCE (%) Reference SC OC TiO CYC-B11 I =I 20.1 0.74 77 11.5 [141] TiO YD2-o-CB Co(bby) 17.7 0.93 74 12.3 [142] 2 3 TiO Y123 Spiro-OMeTAD 9.5 0.98 77 7.1 [143] SnO N3 I =I 7.3 0.7 60 3.0 [130] ZnO N719 I =I 15.2 0.69 50 5.2 [144] 123 Mater Renew Sustain Energy (2015) 4:11 Page 15 of 25 11 Fig. 15 Schematic diagram explaining the principle of photocatalysis. Reproduced from Ref. [155] with permission from The Royal Society of Chemistry When the photons have higher energies greater than the (i) the band gap or energy separation is sufﬁcient or larger band gap of the photocatalyst material, the electrons from than the energy required for the desired reaction; (ii) the the valence band (VB) are excited to the conduction band redox potentials of the electron and hole corresponding to (CB) forming an electron–hole pair. (2) The photogener- their valence and conduction band are suitable for inducing ated carriers (electrons and holes) have the tendency to redox processes; (iii) the reaction rates of the redox pro- recombine on the surface/bulk of the semiconductor. On cesses are faster than the electron–hole pair recombination the other hand, the electron–hole pairs may also get sepa- rate [151]. rated in a surface space charge region. If the diffusion of Moreover, the additional bottleneck for solar energy the electrons and holes is not hindered by any trap states or conversion by photocatalysis is that most metal oxide- defect states, then the charge carriers would eventually based photocatalyst materials are wide-band-gap semi- reach the surface to trigger chemical reactions by charge conductors. Wide-band-gap semiconductors possess elec- transfer from the photocatalyst to an adsorbate. (3) Finally, tronic band gaps around or larger than 3 eV and therefore the reduction reaction occurs when the photogenerated their performance is conﬁned to the small UV region of the electrons interacts with the absorbed molecules on the solar spectrum. Therefore, the quest for ﬁnding efﬁcient semiconductor (photocatalysts) surface. To facilitate the photocatalysts responsive to visible light took the limelight. electron transfer from the photocatalyst to the adsorbate, Approaches such as doping or development of new mate- the conduction band (CB) minimum of the photocatalyst rials suitable such as oxynitrides took the center stage must be higher than the reduction potential of the adsor- [152–154]. One major drawback of employing dopants is bate. Similarly, the photogenerated holes could generate that they often act as trap states serving as recombination strong oxidizing agents like OH radicals by interacting centers and eventually resulting in performance degrada- with the adsorbed molecules on the surface. Here, the tion over a period of time. On the other hand, dye valence band (VB) maximum of the photocatalyst must be adsorption onto the photocatalyst surface is normally lower than the oxidation potential of the adsorbate for considered the second most essential component for pho- efﬁcient hole transfer [148–150]. tocatalytic dye degradation. One of the major factors that determines the dye adsorption onto the photocatalysts Material and electronic properties required surfaces is the surface area [155]. for photocatalysts From the electronic perspective, efﬁcient diffusion of photoexcited charge carriers to the surface with less The development of efﬁcient photocatalysts for high recombination is a basic requisite of any good photocata- chemical conversion efﬁciency solely relies on the means lyst. This process aids the development of speciﬁc surface to suppress back-electron transfer or electron–hole pair sites to exhibit preferential oxidation or reduction chemical recombination process. Therefore, the electron–hole pair reactions. Several factors determine the efﬁcacy of charge generated can be efﬁciently used for the photocatalytic separation and direction selectivity of charge carriers, purpose, provided they exhibit the following properties: which include (i) band structure, (ii) polarization in case of 123 11 Page 16 of 25 Mater Renew Sustain Energy (2015) 4:11 photocatalysts for efﬁcient and stable long-term performance. Titanium dioxide as photocatalysts TiO is widely employed as a photocatalyst in dye wastewater treatment, mainly due to its capability to gen- erate a highly oxidizing electron–hole pair. Moreover, it has good chemical stability, non-toxicity, and long-term photostability [160–162]. The wide band gap (Eg [ 3.2 eV) of TiO limits its potential, because only high energy light in the UV region with wavelengths \387 nm can instigate the electron–hole separation pro- cess [156, 163]. Therefore, developing a photocatalyst that Fig. 16 Comparison of recombination processes of photogenerated can efﬁciently harness the energy from natural sunlight, carriers within a anatase and b rutile structure. Reproduced from Ref. i.e., from the visible region, is one of the major challenges [170] with permission from the PCCP Owner Societies in this ﬁeld [164]. Numerous modiﬁcations of the structure of TiO have been made to achieve the following: (i) de- ferroelectric materials, and (iii) electrostatic potentials in crease the band gap energy to harness the photons from the the surfaces as a function of charged adsorbate present on visible region; (ii) increase the efﬁciency of electron–hole them [156–158]. Directed charge transport can also be production; and (iii) augment the absorbency of organic facilitated by the electronic band bending present in the pollutants onto TiO by appropriate surface modiﬁcations surface or near surface region of the photocatalysts. Upon [148, 149, 165]. Doping is one of the means to achieve the photoexcitation, the band bending provides sufﬁcient space above-mentioned characteristics. Metal ions of noble charge region, assisting the charge separation of photo- metals (Pt, Pd, Ag, and Au) [166] and transition metals (Cr, generated carriers and also aiding the directional diffusion Cu, Mn, Zn, Co, Fe, and Ni) [167] are used as dopants of the electrons to the surface; which in turn enhances the [168]. Even non-metals such as C, N, S, and P are used for photocatalytic activity of the material. The extent of the this purpose [160, 161, 163, 169]. Transition metals are space charge region is determined by the amount of doping used as an alternative to noble metals to reduce the cost. and the dielectric constant of the material. Furthermore, the Fe-doped TiO has been found to exhibit dye degradation size of the catalyst particle also plays a major role in efﬁciency of about 90 % [163]. determining the band bending at the surface. The catalyst particle should be larger than the space charge layer, The difference in photocatalytic activity otherwise there would be no signiﬁcant potential drop between anatase and rutile TiO toward the surface. The width of the space charge layer L sc depends on the materials properties and surface potential In general, anatase TiO usually exhibits higher photocat- Vs, which in turn depends on the surface charges [159]: alytic activities than rutile TiO [170, 171]. The perfor- 1=2 mance improvement arises from the fundamental 2eV L ¼ L ; SC D difference in the electronic properties between them. kT Anatase TiO is an indirect band gap semiconductor, where L is the Debye length, K is Boltzmann constant and whereas rutile belongs to the direct band gap semicon- T is the temperature. ductor. Anatase structure exhibits longer lifetime of pho- On the other hand, materials with high electric constant togenerated carriers (holes and electrons) than direct band increase the width of the space charge region and hence it gap rutile structure. The reason is that in anatase TiO , the has a direct inﬂuence on the amount of photogenerated direct transitions of photogenerated electrons from the carriers extracted to the surface by means of the band conduction band (CB) to valence band (VB) is not possible. bending or surface potential. It is noteworthy to mention Furthermore, anatase exhibits lower average effective mass that the band bending supports either holes or electrons of photogenerated electrons and holes when compared to transport to the surface depending on the direction of the rutile or brookite structure. The lower effective mass potential, thereby the opposite charge carrier is trapped in enables the rapid transport of photogenerated carriers from the bulk of the material decreasing the photoactivity of the interior to the surface of anatase TiO , thereby resulting the catalysts. Hence, it is very much and equally impor- in lower recombination rate and eventually leading to tant to remove the oppositely charged carriers from the higher photocatalytic activity than rutile structure. 123 Mater Renew Sustain Energy (2015) 4:11 Page 17 of 25 11 Table 3 List of highly efﬁcient ZnO- and TiO -based photocatalysts Catalyst Catalyst Reaction conditions Decolorisation Inference/remarks Reference loading efﬁciency -1 P-TiO 0.25 g L Initial concentration of methylene blue 90 % Photocatalytic activity is [180] -5 -1 (MBu): 1.2 9 10 mol L affected by calcination temperature Irradiation time: 40 min UV irradiation 6 W medium-pressure Hg lamp Optimum calcination k = 254 nm temperature is 700 C P:Ti atomic ratio = 0.01 Improved anatase crystallinity -1 P-Doped 0.2 g L Initial conc. rhodamine B (RhB): 12 ppm 90.3 % under natural Un-doped TiO showed rutile [162] TiO sunlight 98.9 % under phase at 800 C Irradiation time: 40 min UV irradiation Visible irradiation: sunlight UV irradiation: 500 W Hg lamp Stirring 3 h for absorption/desorption equilibrium -1 N-TiO 0.2 g L Initial conc. monoazo (ReactiveRead) 100 % ReR 77 % ReBI No change of anatase to rutile [165] ReR, diazo (Reactive Black) ReBI and ratio on N-doping. Remained 100 % DGr poliazodye (Direct Green) DGr :5 ppm constant at 90:10 Irradiation time: 300 min Adsorption capacity is higher due to nitrogen doping Radiation intensity of about 385 W/m for visible light and 0.09 W/m for UV -1 WOx–TiO 1.0 g L Initial conc. Acid Orange 7 (AO ) and 100 % Dye absorbed on WOx–TiO [181] 2 7 2 methyl orange (MeO): 25 ppm surface decolorized and aromatic ? aliphatic acid Irradiation time: 240 min for AO intermediates formed 300 min for MeO visible irradiation: 1000 W halogen lamp short-wavelength components (\420 nm) of the light were cut off using a cutoff glass ﬁlter -1 TiO 0.5 g L Initial conc. MeO: 2 ppm 77.19 % Anatase nanocrystals showed [182] good photocatalytic activity Irradiation time: 540 min in the degradation of UV irradiation: two 6 W ultraviolet light methylorange bulbs (light bulb k = 360 nm) -1 ZnO 0.5 g L Initial conc. Basic Blue 11 (BB-11): 100 % Hydroxyl radical formation [183] 50 ppm better at higher pH Irradiation time: 24 h Visible irradiation: 2 9 15 W visible lamps -1 ZnO 1.25 g L Initial conc. RemBBu(R) RemBl(B): 100 % RemBBu(R), Oxygen important to scavenge [184] 50 ppm RemBl(B): total photo-generated electrons organic carbon (TOC) and prevent electron–hole Irradiation time: 60 min removal: 80 % pair recombination UV irradiation: 125 W Philips Hg lamp RemBl(B); 90 % Toxicity tests showed ZnO not k [ 254 nm pH: 5.0 RemBBu(R) efﬁcient for toxic removal of RemBBu(R) Prolonged degradation increased toxicity -1 ZnO vs. 2.0 g L Initial conc. RemR(F-3B): 150 ppm 100 % TiO /254 nm Comparison of efﬁciency of [185] TiO UV TiO and ZnO photocatalysts 2 2 Irradiation time: 60 min in various parameters: pH, *90 % TiO /365 nm UV irradiation: 6 9 6 W UV lamps light intensity, initial UV k = 365 and 254 nm, intensity 1.85 and concentration, catalyst -2 *95 % ZnO/254 nm 1.65 mWcm loading UV 100 % ZnO/365 nm UV 123 11 Page 18 of 25 Mater Renew Sustain Energy (2015) 4:11 Table 3 continued Catalyst Catalyst Reaction conditions Decolorisation Inference/remarks Reference loading efﬁciency -1 ZnO-TiO 0.5 g L Initial conc. CoR: 5 ppm 100 % Photocatalytically inactive [186] composites Zn TiO and larger band gap 2 4 Irradiation time: 10 min of ZnTiO formed at temp. UV irradiation: 30 W UV lamp [680 C during calcination under O atmosphere calcination 2 ZnO with TiO enhanced temp.: 420 C photocatalytic degradation Adapted from Reference [10] Fig. 17 a Schematic showing the surface plasmon decay processes in overcome the Schottky barrier (/ / - v are injected into SB = M s) which the localized surface plasmons undergoes radiative decay via the conduction band of the adjacent semiconductor, where / is the re-emitted photons (left) or non-radiative decay via excitation of hot work function of the metal and v is the electron afﬁnity of the electrons (right). b Electrons from occupied energy levels are excited semiconductor. Reprinted with permission from Ref. [187]. Copyright above the Fermi energy (Plasmonic energy conversion). c Hot 2014 Nature Publishing Group electrons generated by the plasmons with sufﬁcient energy to Moreover, the electron afﬁnity of anatase is higher than UV and visible light [178]. Other MOSs such as vana- rutile [172]. Therefore, photogenerated conduction elec- dium oxide, tungsten oxide, molybdenum oxide, indium trons will ﬂow from rutile to anatase and this factor is oxide, and cerium oxide have also been studied, but their likely to be the driving force for the increased photoactivity performance is found to be inferior compared to titanium of anatase–rutile composite materials [172]. Comparison of dioxide and zinc oxide [10, 179]. The performance of the recombination processes of photogenerated carriers ZnO- and TiO -based photocatalysts is listed compre- within anatase and rutile structure is shown in Fig. 16. hensively in Table 3. Zinc oxide as a photocatalyst Recent advances in photocatalysts: plasmon-assisted photocatalysis ZnO has a wide band gap (3.2 eV) and the unique elec- trical and optoelectronic property has made it a potential As discussed earlier, the electron–hole separation is of candidate as a photocatalyst. Studies have shown that the paramount importance for realizing higher conversion performance of ZnO under visible light is much more efﬁciencies in photovoltaic and photocatalytic devices. efﬁcient than TiO [173–175]. Though it is highly Employing plasmonic technology for energy conversion is effective under the inﬂuence of UV light [9, 176], with found to be a promising alternative to the conventional suitable physio-chemical modiﬁcations or by doping, ZnO electron–hole separation in semiconductor devices. This can be used as a visible light photocatalyst. Apart from technique involves generation of hot electrons in plasmonic this, the usage of higher-intensity (500 W) visible light is nanostructures by means of electromagnetic decay of sur- found to increase the photocatalytic activity of the ZnO face plasmons [187–190]. The working principle of the nanoparticles [177]. ZnO photocatalyst is also found to be plasmonic energy conversion at the semiconductor inter- better than SnO , CdS and ZnS for dye degradation under face is depicted in Fig. 17. When the metal nanoparticle 123 Mater Renew Sustain Energy (2015) 4:11 Page 19 of 25 11 Fig. 18 a and b TEM images of the Au/TiO nanostructure. c Comparison of decomposition of MO dye by commercial TiO NPs (P25) and Au/ TiO nanostructure prepared by different methods. Reprinted (adapted) with permission from Ref [195]. Copyright (2014) American Chemical Society (plasmon) is illuminated with highly energetic photons, hot activity of the Au/TiO depend on numerous variables. The electrons are generated in conditions of non-radiative intensity of the SPR strongly relies on the shape of the Au decay. Hot electrons whose energies are sufﬁciently higher and TiO NPs. Other factors include (i) dielectric constant than the work function of the material get injected into the of the surrounding medium, (ii) quantum mechani- neighboring semiconductor, thereby producing a pho- cal/electronic interactions between the ligands (stabilizers) tocurrent. This interesting phenomenon caught the atten- and the nanoparticles, and (iii) monodispersity of the NPs tion of the researchers and led to the development of [194]. Au/TiO photocatalysts prepared by depositing pre- NMNPs (noble metal nanoparticles)/semiconductor synthesized colloidal Au nanoparticles onto TiO nanostructures as photocatalysts. Among the various nanocrystals of precisely controlled size and morphology structures thus produced with this combination, Au/TiO through a delicately designed ligand-exchange method nanostructures show promising prospect, as the surface resulted in Au/TiO Schottky contact with low energetic plasmon resonance (SPR) effect in these structures charge transfer barrier. The photocatalysts thus obtained by enhances the photoactivity of titania under visible light. In this strategy showed superior activity compared to con- this process, the photogenerated electrons possess negative ventionally prepared photocatalysts in dye decomposition potential higher than that of the conduction band of the under visible-light illumination [195]. Figure 18 shows that TiO ; thereby, the photogenerated electrons transfer efﬁ- the rate of decomposition of the dye is signiﬁcantly higher ciently from excited Au NPs to TiO NPs [189–191]. For than that of the commercial/pure TiO NPs. Thus, the mode 2 2 instance, in the event of degradation of the pollutant of deposition of the Au and TiO NPs signiﬁcantly affects 4-chlorophenol (4CP) using P-25 titania (commercial the performance of the photocatalysts. Therefore, under- TiO ), the Au NPs signiﬁcantly enhanced the catalytic standing the complex processes affecting the photocatalytic activity by 80 % [192]. The photocatalytic mineralization performance is indispensable for the rational design of the of 4-CP exhibited by Au/TiO is higher than Pt/TiO [ ideal noble metal-modiﬁed metal oxide semiconductor 2 2 Ag/TiO [ TiO [193]. Factors affecting the photocatalytic photocatalysts. 2 2 123 11 Page 20 of 25 Mater Renew Sustain Energy (2015) 4:11 their evolution (2012–2050). J Renew Sustain Energy 5, 023112 Conclusion (2013) 6. Razykov, T.M., Ferekides, C.S., Morel, D., Stefanakos, E., In the future, the photovoltaic market depends not only on Ullal, H.S., Upadhyaya, H.M.: Solar photovoltaic electricity: our ability to increase power conversion efﬁciencies, but current status and future prospects. Sol. Energy 85, 1580–1608 (2011) also on the stability of the devices. Moreover, the pho- 7. Norris, D.J., Aydil, E.S.: Materials science. Getting Moore from toanodes in DSSCs require high sintering temperature, solar cells. 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Publisher: Springer Journals
Copyright: Copyright © 2015 by The Author(s)
Subject: Material Science; Materials Science, general; Renewable and Green Energy; Renewable and Green Energy
ISSN: 2194-1459
eISSN: 2194-1467
DOI: 10.1007/s40243-015-0054-9
Publisher site: See Article on Publisher Site

Abstract

Mater Renew Sustain Energy (2015) 4:11 DOI 10.1007/s40243-015-0054-9 REVIEW PAPER Metal oxide semiconducting interfacial layers for photovoltaic and photocatalytic applications 1 2 3 1 • • • • Naveen Kumar Elumalai Chellappan Vijila Rajan Jose Ashraf Uddin Seeram Ramakrishna Received: 6 November 2014 / Accepted: 7 April 2015 / Published online: 3 July 2015 The Author(s) 2015. This article is published with open access at Springerlink.com Abstract The present review rationalizes the signiﬁcance Keywords Photovoltaics Metal oxide semiconductor of the metal oxide semiconductor (MOS) interfaces in the Interfacial layers Renewable energy Photocatalysts ﬁeld of photovoltaics and photocatalysis. This perspective considers the role of interface science in energy harvesting using organic photovoltaics (OPVs) and dye-sensitized Introduction solar cells (DSSCs). These interfaces include large surface area junctions between photoelectrodes and dyes, the Growing economies and increasing population demand interlayer grain boundaries within the photoanodes, and the more energy in the coming years. The prospect of global interfaces between photoactive layers and the top and warming and limited fossil fuel reserves necessitates radi- bottom contacts. Controlling the collection and minimizing cal changes in our global energy production and con- the trapping of charge carriers at these boundaries is crucial sumption patterns. Primary supply of sustainable and eco- to overall power conversion efﬁciency of solar cells. friendly energy is one of the major challenges of the Similarly, MOS photocatalysts exhibit strong variations in twenty-ﬁrst century. A recent report on 2030 Energy Out- their photocatalytic activities as a function of band struc- look by BP points that an additional 1.3 billion people will ture and surface states. Here, the MOS interface plays a grow into new energy consumers by 2030 [1]. Exxon’s vital role in the generation of OH radicals, which forms the 2040 Energy Outlook projects 85 % growth in global basis of the photocatalytic processes. The physical chem- electricity demand during 2010–2040 [2]. Emerging non- istry and materials science of these MOS interfaces and OECD countries alone will experience a 150 % surge in their inﬂuence on device performance are also discussed. electricity demand. However, to have such quanta of energy, the current energy growth of 1.6 % per year would require at least 35 years; therefore, a crisis is inevitable. This increased energy demand is one part of the story; the & Naveen Kumar Elumalai other part is depleting natural resources, increased pro- n.elumalai@unsw.edu.au duction cost and high environmental concerns such as Chellappan Vijila global warming due to excessive use of fossil fuels. Over c-vijila@imre.a-star.edu.sg 85 % of the primary supply in the present-day energy mix is contributed by fossil fuels, thereby putting the life sus- School of Photovoltaic and Renewable Energy Engineering, tenance at an increased risk in the planet [3]. To point out a University of New South Wales, UNSW, Sydney, NSW 2052, Australia consequence of increased energy production cost, many gas wells generate 80–95 % less gas after just 3 years Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), contrary to the predictions that their lifetime would be 3 Research Link, Singapore 117602, Singapore 40 years. Given this swift weakening in natural gas pro- Universiti Malaysia Pahang, 26300 Pahang, Malaysia duction from newly drilled wells, it would be essential to drill 7200 wells per year at a cost of 42 billion dollars, to National University of Singapore, Singapore 117576, sustain the present level of natural gas production [4]. All Singapore 123 11 Page 2 of 25 Mater Renew Sustain Energy (2015) 4:11 these concerns focus our attention towards clean, sustain- other hand, the electrochemical properties of the MOS such able, and zero cost sources of energy; i.e., the solar energy as band edge energies determine their success as and its conversion into electrical energy as the most con- photocatalysts. venient for use in modern life. A total of 36,000 TW solar This review emphasizes on the application of MOSs as energy strikes the Earth. Assuming an efﬁciency of 25 %, a electrodes and interfacial layers in the technologies that solar cell farm of area *367 km 9 367 km in the Sahara involve photon-assisted physiochemical processes. In desert would meet the projected energy demand. For Sect. 2, the role of MOSs in photovoltaics—dye-sensitized comparison, this area is only 0.3 % of 9.4 million km of solar cells (DSSCs) and organic solar cells (OSCs)—are the Sahara desert. Therefore, the sun could be a single discussed. The interfacial processes and energetics solution to all our future energy needs [5–7]. involved in the charge injection and extraction properties Similarly, growing population has also increased the use of these materials are elaborated. In Sect. 3, the effect of of dyes in many industries such as textile, furniture, MOSs in photocatalytic systems is addressed focusing on chemical and paint. The dumping or accidental discharge dye degradation. Recent advances in photocatalysis of dye wastewater has triggered a substantial amount of involving plasmon/metal oxide interface is also discussed. environmental and health problems, urging researchers globally to develop universal methods to treat dye wastewater efﬁciently. Until now, orthodox methods such Photovoltaics as coagulation, microbial degradation, biosorption, incin- eration, absorption on activated carbon, ﬁltration and sed- Recent advances in solar energy conversion technologies imentation have been employed to treat dye wastewater [8, based on organic semiconductors as light-harvesting layer, 9]. Recently, an approach called advanced oxidation pro- such as dye-sensitized solar cells and organic solar cells, cess was developed to treat dye wastewater treatment [10]. employ MOS nanostructures for efﬁcient charge extraction This method involves the process of generating strong OH and transportation between the electrodes and organic radicals for breaking down the complex molecules. Solar molecules. The transition MOSs are well known for their energy serves as the cheapest and efﬁcient source of energy ability to exchange charges with condensed molecules, for generating these radicals through the process of pho- making them a viable and cost-effective candidate to be tocatalysis. Thus, proper engineering and optimization of used in the photovoltaic devices. the materials can provide viable and efﬁcient ways to Solar cells are classiﬁed into different schemes based harness this abundant resource for photovoltaic and pho- either on the historical evolution or on their principles of tocatalytic applications. operation. The class of solar cells based on a p–n junction Semiconductors, deﬁned as materials with band gap is the ﬁrst of its evolution and, therefore, are typically energy B5 eV, are essential to absorb solar energy for called ﬁrst-generation solar cells [11, 12]. Semiconductors, enabling the above tasks; a multibillion dollar semicon- either elemental such as Si or Ge or compounds such as ductor industry is in operation with diverse types of GaAs or InP, are materials of choice to build p–n junction semiconductors including elemental and compound semi- solar cells. Photoexcited carriers in the p–n junction are conductors. Among them, metal oxide semiconductors separated into mobile carriers by the built-in-electric ﬁeld, (MOSs) represent a domain of relatively inexpensive and or band bending, at the junction between the p- and the environmentally benign class of materials with a diverse n-type semiconductors [12–14]. The photovoltage in the p– range of properties owing to their rich structural diversity. n junction is the difference in quasi-Fermi levels (i.e., the Both natural and synthetic MOSs have diverse applica- band bending) of n-type and p-type regions. A typical tions. The properties of MOS can be tailored in many ways, device consists of a 5 lm-long n-type semiconductor and viz., varied choice of morphologies, introducing oxygen 300 lm-long p-type ones, i.e., the minority carriers in the vacancies, doping, and so on. p–n junction are expected to travel *300 lm for efﬁcient In photovoltaics, MOSs serve as a scaffold layer for charge collection, which requires rigorous control on their loading dyes in dye-sensitized solar cells (DSSCs) and chemical purity [12, 15]. Requirement of such extreme organic–inorganic hybrid perovskites in perovskite solar purity of the semiconductors is one of the major cost cells (PSCs), as well as electron and hole transport layers in limiters of the ﬁrst-generation solar cells. The p–n junc- DSSCs and organic solar cells (OSCs). The function of tions are typically built on single crystalline and poly- scaffold in DSSCs is to facilitate charge separation and crystalline platforms. The latter polycrystalline overcomes charge transport, whereas that of the transport layers is to the cost limitations on chemical purity, however, at the conduct one type of charge carrier block to the other type. expense of the photovoltaic conversion efﬁciency (PCE) Therefore, tailoring their properties is inevitable to develop [14, 16]. Whether or not a p–n junction is made from single high-performing photovoltaic devices using them. On the or polycrystals, inherent limitations between the absorption 123 Mater Renew Sustain Energy (2015) 4:11 Page 3 of 25 11 and electron emission in those crystals put a theoretical metals and are often characterized as a mobile excited limit on the photovoltaic conversion efﬁciency in p–n state. An exciton can be considered as a quasi-particle with junction solar cells, known as a Schokely–Queisser limit. an electron in the conduction band (or lowest unoccupied The Schokely–Queisser limit predicts a theoretical upper molecular orbital, LUMO) and a hole in the valence band limit of 32 % PCE for single-junction (p–n) solar cells (or highest occupied molecular orbital, HOMO). When a [12]. semiconductor (molecule, crystals, or clusters) is anchored The second-generation solar cells are based on the with another material whose conduction band (LUMO) lies charge separation at an interface either between two con- at lower energy, then the exciton dissociates into mobile jugated polymers or a ﬂuorophore molecule conjugated carriers (or free carriers) at the interface of the material with a metal oxide semiconductor. The National Renew- system (Fig. 2). This process is the basis of ESCs. Exam- able Energy Laboratory (NREL) at Colorado, USA, cate- ples of this type of ESCs include organic solar cells gorize them as ‘‘emerging solar cells’’ (Fig. 1). In the third (OSCs), dye-sensitized solar cells (DSSCs), and quantum generation, semiconducting nanocrystals of size in the dot solar cells. Conjugated polymers and/or organic quantum conﬁnement region, known as quantum dots, is materials such as PCBM, P3HT, etc., are the materials of used as the light harvester [17]. The quantum dot offers the choice in OSCs. In the DSSCs, a wide-band-gap MOS, possibility of many photoelectrons per single absorbed such as TiO , is anchored to a dye. In the third-generation photon of sufﬁcient energy, thereby uplifting the theoreti- quantum dot solar cells, quantum dots are used as light cal conversion efﬁciency over 60 %. The ﬁrst- and second- harvesters [18–20]. generation solar cells are collectively called ‘excitonic Although an upcoming energy technology, photo- solar cells (ESCs). voltaics—the science and technology of solar cells—has In the ESCs, light absorption results in the generation of steadily progressed. One may note that the p–n junction a transiently localized excited state, known as exciton— solar cells made from single crystals have reached a stage usually, a Frenkel type is formed. These Frenkel excitons of performing with theoretical conversion efﬁciency. On cannot thermally dissociate into free carriers in the material the other hand, performance of ‘‘emerging solar cells’’ in in which it was formed. Moreover, excitons are the char- the second and third generation is relatively inferior. acteristics of semiconductor analogs to Fermi ﬂuids in Compared to the ﬁrst-generation solar cells, these emerging Fig. 1 Best research-cell efﬁciencies of different types of solar cells. This plot is courtesy of the National Renewable Energy Laboratory, Golden, CO. (2015) 123 11 Page 4 of 25 Mater Renew Sustain Energy (2015) 4:11 Fig. 2 Schematic illustration of an excitonic solar cell solar cells offer ease of fabrication, ﬂexibility, lower cost, and higher performance efﬁciency. Although p–n junction solar cells have the advantage of high PCE ([20 %) and long lifetime (25 years), ESCs have improved enormously in the past few years demonstrating PCEs as high as 12 % and lifetimes of 10 000 h [21–23]. Progress in photovoltaics has become indispensable to improve the performance and cost-effectiveness of har- nessing the solar energy. As a function of this driving factor, intense research works are carried out across various components of a photovoltaic device. Among them, engi- neering the nanometer (electron) and micrometer (photon) scale interfaces among the crystalline domains is impera- tive for efﬁcient charge transport, as these domains con- stitute the interfacial layers in the solar cells. The interfacial layers include the following: (1) inter-percolated high surface area junctions between photoelectron donors and acceptors, (2) the interlayer grain boundaries existing within the absorber material, and (3) the interfaces between Fig. 3 Existing challenges in the ﬁeld of excitonic solar cells the top and bottom contacts in conjunction with the pho- toactive layers. Optimizing the charge carrier transport discuss the physical chemistry and materials science of across these interfaces is pivotal for the efﬁciency and these MOS interfaces and their impact on device lifetime of the photovoltaic device [24]. The existing performance. challenges in the ﬁeld of excitonic solar cells is represented in Fig. 3. Organic solar cells In this section, the application of MOSs in two main groups of solar technologies that are approaching or have Organic solar cells (OSCs) have attracted immense exceeded 10 % solar power conversion efﬁciency are dis- research interest in the past decade owing to easy fabri- cussed, i.e., dye-sensitized solar cells (DSSCs) and organic cation and low cost [25]. The quest for improvement in solar cells or organic photovoltaics (OPVs). Here, we also 123 Mater Renew Sustain Energy (2015) 4:11 Page 5 of 25 11 power conversion efﬁciency (PCE) of OSCs has been fullerene derivatives along with the electron afﬁnities and actuated by the development of novel photoactive materials ionization energies of high or low-work-function MOS and device architectures [26–28]. Numerous donor–ac- employed in OSCs [42]. These MOSs provide the basis for ceptor (D–A) polymers have been synthesized by varying developing highly efﬁcient and stable ohmic contacts by the highest occupied molecular orbital (HOMO) and lowest means of energy-level bending, vacuum-level shifting and unoccupied molecular orbital (LUMO) levels of the donor Fermi-level pinning at the polymer–electrode interfaces. and the acceptor materials [27, 29–31]. The enhancement Furthermore, the use of oxide interlayers evades the direct in short circuit currents (J ) and open circuit voltages contact between the photoactive layer and electrodes, SC (V ) achieved by means of band gap and interfacial where high densities of carrier traps or adverse interface OC engineering resulted in high PCE *10 %. [25] Further- dipoles hamper efﬁcient charge collection. Moreover, MOS more, development of novel printing and coating processes interfacial layers play a dominant role for developing have been developed leading to roll-to-roll processing of ohmic contacts to maximize the V , because lower built- OC organic solar cells [32–34]. During this course of devel- in potential eventually leads to an increase in dark current opment, transition metal oxide semiconductors (MOS) as well as carrier recombination. have gained profound attention in OSCs owing to their ability to transport/extract charge carriers efﬁciently and Metal oxide semiconductors for OSCs solution processibility which is well suited for roll-to-roll high-volume production. Metal oxide semiconductors (MOSs) for the OSCs can be Organic solar cells (OSCs) mostly employ bulk hetero- p-type and n-type materials, contingent on the position of junction (BHJ) device structure in which the photoactive the conduction band and valence band. For an n-type blend comprises an electron-donating polymer and an material, electron transfer from the LUMO of the acceptor electron-accepting fullerene derivative in a pattern of to the conduction band (CB) of the MOS is the requisite. nanoscaled interpenetrating networks. Fabrication of ohmic For a p-type contact material, the valence band (VB) of the contacts is very important in OSCs, but it is not as MOS is required to match the HOMO of the polymer. The straightforward as that in inorganic solar cells. OSCs wide band gap of the interface materials also serves as a require speciﬁc materials for this purpose with appropriate barrier for carriers of the other sort, thus improving the interface engineering. Poor ohmic contacts with transparent carrier selectivity of the contacts [40, 43–47]. conducting oxides (TCO) or metallic electrodes arise due The main roles of interface materials are: to the (i) misalignment of energy levels or mismatched 1. To align/adjust the energetic barrier height between work function [35, 36], (ii) the formation of interfacial the photoactive layer and the adjacent electrodes. dipoles [37, 38] and (iii) interfacial trap states [39]. Various 2. To materialize a selective contact for carriers of one charge-extracting interlayers have been employed between sort (either holes or electrons). the active layer and the electrodes to develop good and 3. To control the polarity of the device (to make normal efﬁcient ohmic contacts. Among the various interfacial or inverted device structure) as shown in Fig. 5. materials used, transition metal oxide semiconductors 4. To prohibit a physical or chemical reaction between (MOS) are considered as potential candidates owing to the polymer and electrode. their high environmental stability, superior optical trans- 5. To serve as an optical spacer. parency and facile synthesis routes. Open circuit voltage V of OSCs is determined by the OC Cathode interfacial layers for OSCs energy-level alignment between the donor and the acceptor in devices with ohmic contacts; if not, it is determined by the work function difference of the contact electrodes [26, Based on the device architecture (i.e., normal or inverted) 40]. On the other hand, short circuit current (J )is as shown in Fig. 5, the cathode interfacial layer lies in SC determined by the amount of photogenerated carriers pro- conjunction with the low-work-function metal (top elec- duced in the active layer upon illumination and the charge trode) or at the bottom adjoining the TCO electrode. Ini- separation efﬁciency across the photoactive layer. Fill tially, alkali metals or related compounds are used to make factor is determined by the factors such as series resistance, ohmic contacts to the electron acceptors in the bulk shunt resistance, and charge recombination/extraction rate heterojunction/photoactive layer. Cesium carbonate (Cs 2- in the device. Finally, the performance efﬁciency (PCE) is CO )[48] and lithium ﬂuoride (LiF) were used for this determined by the product of Voc, Jsc, and ﬁll factor; purpose owing to their low work function (\3.0 eV). They exhibited good electron injection properties and enhanced which is then normalized to the incident light intensity usually 1 sun at AM 1.5 G [41]. Figure 4 shows the HOMO the V of the device effectively [49]. However, degra- OC and LUMO levels of various donor polymers and acceptor dation issues such as oxidation of alkali metal compounds 123 11 Page 6 of 25 Mater Renew Sustain Energy (2015) 4:11 Fig. 4 Schematic view of the energy levels of metal oxides and orbital energies of some of the organic components used in OSCs. Reprinted (adapted) with permission from ref [47]. Copyright (2011) American Chemical Society Such characteristics make the n-type semiconducting oxi- des an effective alternative to low-work-function metals used as cathode contacts. The proﬁciency of these oxides is demonstrated in both conventional and inverted device geometries [51–56]. ZnO is presently the most widely used electron-trans- porting layer (ETL) in OSCs. Its Fermi level aligns well with the LUMO of electron acceptors [57]. Moreover, it also acts as an effective hole blocker owing to its high ionization potential, thereby increasing the shunt resistance of the device [55, 58]. ZnO is transparent to visible light and absorbs in the UV spectrum, serving as a low band ﬁlter for the photoactive layer. To fabricate OSCs with inverted device geometry, ZnO nanoparticles or colloids are spin coated onto TCO substrates. Solution-processed ZnO nanostructures are obtained from precursor solutions containing Zinc salts through methods such as sol–gel [59], solvothermal [60], or a hydrothermal process [61]. More- over, ZnO NPs are readily synthesized from zinc acetate dehydrate and used as ETL [62–64]. Using such an approach, inverted poly (3-hexylthiophene) (P3HT) cells with a PCE of 4 % have been demonstrated [65]. The Fig. 5 Schematic representation of a normal and b inverted device synthesized ZnO nanostructures have a lot of defect states structure of OSCs [98]. Reproduced by permission of the PCCP Owner Societies in them. The small diameter of the ZnO NPs possesses a large fraction of dangling bonds, giving rise to high density over a period of time leads to poor stability [50]. Therefore, of gap states. Thus, elimination of localized energy states the use of transition MOS such as ZnO and TiO with work in the band gap of the charge transport layers in inverted functions corresponding to the LUMO levels of fullerenes organic solar cells signiﬁcantly augments the operational is considered to be an effective alternative. These MOSs stability of the device in addition to enhanced photovoltaic are well known for their chemical resistance to oxygen and parameters. Therefore, UV exposure is required to improve moisture, optical transparency and solution processibility. the conductivity of ZnO NPs by means of photodoping and 123 Mater Renew Sustain Energy (2015) 4:11 Page 7 of 25 11 defect ﬁlling. UV-ozone (UVO) treatment was also found evaluated and the one with a higher crystallinity was to passivate the surface defect states [57]. With this characterized by a trap depth lower by a factor of two, approach, PDTG-TPD:PC BM-based devices with ZnO when compared with the device annealed at lower tem- NPs as ETL exhibited enhanced device performance with perature [67]. PCE *8% [66]. However, this UV exposure method is Under the testing protocol ISOS-L-1, the devices A not sufﬁcient for optimum device performance. Another (annealed at 240 C) and B (annealed at 160 C) retained approach to achieve optimum device performance was by 64 and 48 % of their original efﬁciency after 400 h of using in P3HT:PCBM-based devices by increasing the ZnO constant illumination, respectively (refer Fig. 7)[67]. nanostructure crystallinity and annealing temperature. Moreover, ZnO nanowires were planted across the ZnO Altering the crystallinity by annealing the ZnO at two nanoparticles to increase the electron lifetime, decrease temperatures, viz. 160 and 240 C, produced a difference recombination, and improve charge collection at the cor- in the density of localized energy states in the band gap of responding electrodes. Reports have shown that incorpo- ZnO. The trap depth in the device annealed at 240 Cis rating electrospun ZnO nanowires could effectively lesser when compared with that annealed at 160 C, as increase the carrier lifetime by twofold [54]. shown in Fig. 6 [67]. The devices fabricated using the ZnO ZnO is also used to form the interconnecting unit in nanostructures annealed at 240 C showed remarkably tandem OSCs. Such an interconnecting unit, basically a higher power conversion efﬁciency (PCE) and IPCE val- tunneling p–n junction, is sandwiched between the two ues. The depth of electronic traps in the two devices was BHJ cell stacks, thereby forming a hole–electron recom- bination zone [68]. In this zone, the Fermi levels of the HTL and the ETL are well matched to minimize the V OC loss in a tandem cell. Thus, ZnO is widely applied in OSCs for research and commercial purposes [68]. Similarly, TiO is also an n-type and wide gap semi- conductor (E * 3.2 eV) with its conduction band minima composed of the Ti 3d band and its valence band maxima composed of the O 2p states [69–73]. In OSCs, TiO ﬁlms are usually processed by sol–gel process; therefore they are in amorphous phase rather than crystalline. The solution precursor is usually prepared using titanium isopropoxide along with solvent additives. TiO ﬁlms are typically deposited by spin coating at optimal speed followed by annealing up to *150–200 C, similar to the synthesis of Fig. 7 Stability of the inverted devices with ZnO (ETL) and MoO (ETL) under constant illumination conditions tested under Protocol Fig. 6 Trap depth (D) in devices A (160 C) and B (240 C) ISOS-L-1. a ZnO with low degree of localized states b ZnO interlayer calculated from ln J vs 1/T curves. Inset shows the variation of trap with high degree of localized states or larger trap depth [57]. depth (D) as a function of U [67]. Reproduced by permission of the Reproduced by permission of the PCCP Owner Societies PCCP Owner Societies 123 11 Page 8 of 25 Mater Renew Sustain Energy (2015) 4:11 sol–gel ZnO discussed in the previous section. OSCs fab- well as the resistance at the photoactive layer/anode ricated with sol–gel TiO ﬁlms have been demonstrated in interface [85]. Besides, MoO HTL also serves as an x 3 both conventional and inverted cells. In conventional cells, optical spacer for improving light absorption, thereby the incorporation of TiO as an ETL exhibited enhanced enhancing the photocurrent [86–88]. Molybdenum oxide is J and FF when compared with devices using aluminum widely used as hole injection material and was considered SC electrodes. OSCs fabricated with solution-processed TiOx as a p-type semiconductor initially. Until recently, UPS ﬁlms serving as ETL and PCDTBT:PC BM blend as studies have exposed that it is of n-type semiconductor and photoactive layer, results in PCE as high as 6 % [74]. the charge transport occurs via the Fermi level being pin- The improvement of PCE was attributed to the improved ned with the valence band of the polymers. Moreover, electrical coupling with PC BM and the improved light vacuum-deposited/thermally evaporated MoO gives the 71 3 harvesting [75]. Here, TiO acts as an optical spacer pro- advantage of precise thickness control in the nanoscale viding more absorption cross section at the photoactive range of about 1–2 nm. Reports have revealed that the layer. TiO was also found to exhibit strong dependence on oxygen deﬁciency in e-MoO results in defect states in 2 3 UV illumination [76, 77]. The UV-activated TiO ﬁlms are energy bands, which raises the Fermi level closer to the shown to degrade much rapidly under continuous illumi- conduction band [89, 90]. nation during regular operating conditions. The UV light However, MoO is highly sensitive to oxygen and causes photodoping of TiO where oxygen is chemisorbed moisture; even the trace amounts of oxygen in the nitrogen- at those sites, leading to unfavorable band bending in the ﬁlled glove box during device fabrication are shown to TiO and thereby hindering charge extraction [78]. UV have detrimental effects on its electronic levels, imposing photodoping is one of the prime issues to be solved to use it severe shortcomings in the device stability [91]. Upon in OSCs as the ETL layer efﬁciently. exposure to ambient conditions (oxygen or air), the work function decreases to 5.3–5.7 eV, which is still adequate Metal oxide semiconductors (MOS) as anode enough to form good ohmic contacts with organic hole interfacial layers transporting materials. Reports have indicated that further reduction of the suboxide results in the growth of gap states The primary requirement of an efﬁcient MOS serving as that would ﬁnally reach the Fermi energy, resulting in the HTL or anode interfacial layer is that its work function metallic behavior of MoO [80, 92]. needs to be high as well as align with the HOMO level of Generally, the hole transport in nanostructured MoO the photoactive polymers [40, 46, 79]. The conjugated layer occurs via the shallow defect states present in its band polymers with high ionization potential cannot form ohmic gap formed as a result of oxygen vacancies [93–95]. These contacts directly in conjunction with the metal electrodes, oxygen vacancies serve as n-type dopants and lead to because of the electron transfer from the metal to the Fermi-level pinning at the photoactive layer–MoO inter- organic photoactive layer. This electron transfer creates a face [96]. The mechanism of the charge transfer process dipole at the interface, leading to reduction in built-in across the MoO interlayer is represented in Fig. 9, which potential of the device. The drop in built-in potential shows that the holes are extracted by injecting electrons increases the series resistance and charge extraction ﬁeld, into the HOMO of the donor (P3HT). Subsequently, the thereby hindering the device performance. The energy holes transferred to MoO hop to the Ag electrode through level interaction at the MOS–organic interface is repre- the shallow defect states generated by the oxygen vacan- sented in Fig. 8 [80]. cies [97]. Numerous vacuum-deposited transition MOS with high Using MoO to replace PEDOT:PSS as the anode work functions, eminent optical transparency, and good interfacial layer (HTL), the resulting devices show com- stability have attracted signiﬁcant research interest. OSCs parable initial performance with much enhanced stability incorporated with these MOSs have demonstrated good as shown in Fig. 10, demonstrating that high-work-func- device performance. Most transition MOSs such as tion n-type MOS can effectively replace PEDOT:PSS in molybdenum oxide, tungsten oxide, and vanadium oxide both conventional and inverted devices [98]. are commonly used in n-type semiconductors for HTL. Reports have shown that a high degree of oxygen These MOSs enable effective Fermi-level pinning and also defects were introduced in the hole-conducting MoO layer -5 increase the built-in potential of the device. The role of by annealing the devices under vacuum (*10 mbar) at these MOSs as HTL is discussed in the following section. nominal temperature (120 C) and time (10 min). The Molybdenum oxide (MoO ) has gained signiﬁcant devices thus fabricated exhibited much higher operational attention for improving device performance and stability in stability, when tested following the ISOS-D-1 (shelf) pro- OSCs [81–84]. The MoO HTL reduces the charge tocol, than control devices annealed conventionally, i.e., in recombination by suppressing the exciton quenching as nitrogen atmosphere. 123 Mater Renew Sustain Energy (2015) 4:11 Page 9 of 25 11 Fig. 8 Energy level at oxide/organic interface. Reprinted with permission from Ref. [80]. Copyright 2012 Nature Publishing Group Fig. 9 Schematic showing the mechanism of hole transport across the molybdenum trioxide (MoO ) interlayer [97]. Reproduced by permission of the PCCP Owner Societies For large-scale production or R2R processing, it is lower than that of the devices made with e-MoO .An necessary to deposit MoO by a solution-processing tech- investigation of better solution-processing conditions and nique instead of vacuum deposition. Reports have shown methods is needed to make it commercially successful [45, that P3HT:PCBM cells with solution-processed MoO 99, 100]. showed a PCE of 3.1 %. Despite the low-temperature Another n-type MOS with high work function is tung- process and acceptable PCE values, aggregation of sten oxide, WO ; similar to MoO , its electronic structure 3 3 s-MoOx is one of the major issues hindering its application is highly determined by its stoichiometry, its crystalline in large-scale processing. However, the work function structure, and processing/deposition conditions. Evapo- values of s-MoOx ﬁlms tend to be lower than that of rated ﬁlms of amorphous WO are generally deﬁcient in e-MoO , thus affecting the quality of the resulting ohmic oxygen, which gives rise to the gap states and n-type contacts. P3HT:PCBM-based devices are made with semiconductivity. Thermally evaporated ﬁlms of WO are s-MoOx, as HTL exhibits a V of 0.55 V which is 50 mV typically oxygen deﬁcient, thereby possessing a large OC 123 11 Page 10 of 25 Mater Renew Sustain Energy (2015) 4:11 105]. The inverted OSC devices fabricated with PCDTBT in the active layer along with TiO as the electron transport layer (ETL) and MoO as the hole transfer layer (HTL) exhibits high Voc of about 91 % with a PCE of 7.2 % as shown in Table 1, owing to low band gap of the polymer and efﬁcient charge transport across the interface [98]. The devices that employ tungsten oxide (WO ) as HTL shows superior performance than vanadium pentoxide (V O )as 2 5 HTL within the same P3HT-based active layer. The V of OC the WO -based device shows signiﬁcant improvement compared to that of the latter owing to reduced charge transport barrier at the interface and lower series resistance [106, 107]. Table 1 shows the photovoltaic parameters Fig. 10 Comparison between the normalized PCEs as a function of obtained for the inverted organic solar cells employing time for PCDTBT: PC BM devices employing PEDOT:PSS and different photoactive layer and interfacial layers. MoO as hole-transporting interfacial layers (HTL). Copyright 2011 Wiley-VCH [98] Dye-sensitized solar cells (DSSCs) amount of gap states. The oxygen deﬁciency also con- O’Regan and Graetzel reported on the dye-sensitized solar tributes to the n-type semiconductivity of WO ; therefore, cell (DSSC) established on the mechanism of novel its electrical properties like work function are also sensitive regenerative photoelectrochemical processes with an efﬁ- to oxygen exposure. The optical band gap of WO of ciency of *7.9 % [73]. Following its success, extensive thermally evaporated ﬁlms is around 3.2–3.4 eV. Oxygen research has been carried out in this ﬁeld to increase the exposure is found to increase further up to 4.7 eV. Its power conversion efﬁciency (PCE) of DSSCs by incorpo- performance as HTL is signiﬁcantly acceptable as PED- rating n-type MOSs such as TiO , ZnO, Nb O , SrTiO , 2 2 5 3 OT:PSS, with the devices exhibiting V of 0.6 V and FF and SnO and their composites as photoelectrode materials. OC 2 of 60 % [44, 101]. Reports have indicated that solution- The wide-band-gap MOSs (Eg [ 3 eV) having suitable processed tungsten oxide with a larger work function band position relative to dye (or photosensitizer) have been -2 increased the efﬁciency to 3.4 % with J * 8.6 mAcm ; used for the fabrication of DSSCs. Owing to the wide band SC V * 0.6 V; FF * 0.6 for a P3HT:PCBM cell. The gap, the MOSs employed for the fabrication of DSSCs OC devices exhibited an enhanced lifetime/stability by main- have absorption at the ultraviolet region. Therefore, pho- taining 90 % of the initial value after being exposed to tosensitizer/dye is responsible for the absorption of light at ambient conditions for nearly 200 h without any encapsu- the visible and near-infrared region. Furthermore, the high lation [102, 103]. These factors make s-WO a viable surface area of nanoporous MOS increases dye loading; candidate than other solution-processed high-work-func- thereby enhancing light absorption leading to improved tion oxides for application in large-scale coating in the R2R performance of DSSCs. In addition to the above-mentioned process. physical characteristics, low cost, natural abundance, and Another n-type semiconducting oxide used as the anode facile synthesis methods of MOS combined with interlayer is vanadium pentoxide, V O . Its band gap is facile solution processibility is another key advantage for 2 5 around 2.8 eV as estimated by UPS and IPES studies [100], the application in DSSCs [71, 112, 113]. revealing that its absorption band partially covers the Among the many wide-band-gap oxide semiconductors absorption band of PC BM. Similar to MoO and WO , (TiO , ZnO, and SnO ) that have been examined as 71 3 3 2 2 the band structure of thermally evaporated V O is highly potential electron acceptors for DSSCs, TiO is the most 2 5 sensitive to the ambient conditions. P3HT:PCBM devices versatile. It delivers the highest efﬁciencies, is chemically -6 fabricated using e-V O (*10 Torr) as an anode inter- stable, non-toxic, and available in large quantities. TiO 2 5 2 layer exhibited an optimum PCE of *3 %. Upon air has many crystalline forms, with anatase, rutile, and exposure, the work function of e-V O further reduces to brookite being the most important ones. The crystal 2 5 5.3 eV along with a signiﬁcant reduction of electron structure of anatase and rutile are based on a tetragonal 4? afﬁnity and increase of defect states. In comparison with symmetry, in which the Ti atoms are sixfold coordinated MoO , research on V O is rudimentary. For a better to oxygen atoms. The main difference between both 3 2 5 understanding of the charge transport mechanism at the structures is the position of the oxygen atoms. In contrast to V O –polymer interface, additional investigations of the rutile, anatase has a smaller average distance between the 2 5 4? interface electronic structures are indispensable [43, 104, Ti atoms; thus, anatase is thermodynamically less stable. 123 Mater Renew Sustain Energy (2015) 4:11 Page 11 of 25 11 Table 1 List of high-efﬁciency OPVs employing MOS interfacial layers Polymer Cathode interlayer Anode interlayer J (mA/cm)V (V) FF (%) PCE (%) Reference SC OC P3HT sol gel-ZnO MoO 10.9 0.57 61.6 3.8 [108] P3HT ZnO NP MoO 12.6 0.63 62 4.9 [109] PCDTBT sol gel-ZnO MoO 10.4 0.88 69 6.3 [110] PCDTBT s-TiO MoO 11.9 0.91 66 7.2 [98] x 3 PDTG-TPD ZnO NP MoO 14.1 0.86 67 8.1 [57] P3HT s- s-TiO WO 7.2 0.59 60 2.6 [111] x 3 P3HT sol gel-ZnO WO 8.19 0.86 67.7 4.8 [106] P3HT sol gel-ZnO V O 10.4 0.56 66 3.8 [107] 2 5 a-PTPTBT sol gel-ZnO VO 11.6 0.82 53 5 [107] P3HT sol gel-ZnO VO 10.1 0.57 67 3.9 [107] The phase transformation from anatase to rutile occurs in connector to the external circuit. The functional principle is the temperature range of 700–1000 C, depending on the similar to photosynthesis: upon photoexcitation, the dye crystallite size and impurities [69, 114, 115]. molecules inject an electron into the conduction band (EC) Rutile has slightly lower indirect band gap (3.0 eV) as of TiO , leaving the dye in its oxidized state (D?, also compared to anatase (3.2 eV), which is attributed to a referred to as dye cation). The dye is restored back to its negative shift of the conduction band in anatase by 0.2 eV. ground state by electron transfer from the redox pair [72]. The bonding within TiO is partly covalent and partly The mechanism of operation of the DSSCs is illustrated in ionic. Therefore, stoichiometric crystals are insulating [71, Fig. 12. 113]. However, a signiﬁcant amount of trap states are The regeneration of the sensitizer by iodide/tri-iodide induced during most synthesis routes, which are due to electrolyte results in the recombination of the conduction oxygen vacancies. These vacancies can also be formed band electron with the oxidized dye. Diffusion of reversibly under reduced pressure and/or elevated temper- e through the nanocrystalline MOS ﬁlm to the substrate 3- ature, which can lead to a variation in conductivity by electrode and diffusion of the oxidized redox species (I several orders of magnitude. The oxygen vacancies cause ions formed by oxidation of I ) through the electrolyte 3? the formation of Ti state, which dope the crystal nega- solution to the counter electrode facilitates both charge tively (n-type). In contrast to other semiconductors of carriers to be transferred to the external circuit, where the similar band gaps (e.g., ZnO), it does not photodegrade energy is transferred to the external load and the regen- upon excitation. On the other hand, TiO is less stable to erative cycle is completed by electron transfer to reduce 3- - UV degradation compared to tin oxide (SnO ), owing to its I to I [20, 71]. It is of critical importance for the high band gap and high work function. However, low functioning of the cell that the injection of electrons into electron mobility (l ) through mesoporous TiO the TiO is many orders of magnitude faster than any n 2 2 2 -1 -1 (*0.1 cm V s ) is a crucial issue and imposes severe recombination (loss) of charge carriers. Moreover, the limitations in enhancing the g of DSSCs closer to the most important recombination process is the direct elec- theoretical limits. The energy levels of the conduction and tron transfer from the conduction band of TiO to the valence bands of the MOS used in DSSCs are shown in redox electrolyte without passing the external circuit [71, Fig. 11. In the standard version of DSSCs, the typical ﬁlm 113]. thickness is 2–15 lm, and the ﬁlms are deposited using Interfacial electron transfer is the process in which the nano-sized particles of 10–30 nm. A double-layer structure excited electron from the LUMO of dye is injected into the can be fabricated, where an underlayer of thickness conduction band of MOS (photoanode) with the rate con- 2–4 lm is ﬁrst deposited using larger (200–300 nm) size stant k [113]. The kinetics of the interfacial electron particles that acts as a light-scattering layer to induce a transfer at the interface strongly relies on the energetics of phototrapping effect [20, 69, 113, 114]. the MOS/dye/electrolyte interface and the density of Dye-sensitized solar cells are based on an MOS nanos- electrons in MOS photoanodes (i.e., Fermi-level of metal tructure that is sensitized with a ruthenium-containing dye oxide). The interfacial electron transfer occurs mostly in a molecule. Different types of MOS photoanodes and their time scale of several picoseconds. Electron injection rate of -12 -1 respective band energies are shown in Fig. 11. A redox [10 s has been reported for several sensitizers and electrolyte and two conducting glass substrates provide the MOS ﬁlms [72, 112, 113]. 123 11 Page 12 of 25 Mater Renew Sustain Energy (2015) 4:11 Fig. 11 Band energies of conduction band (CB) and valence band (VB) of different metal oxides used in DSSCs. Reprinted with permission from ref [72]. Copyright 2001 Nature Publishing Group Injected electron transfers across the mesoporous layer conversion efﬁciency of DSSCs beyond the present record of MOS to the transparent conducting oxide (TCO). The of 11–15 % [69, 114–116]. efﬁcacy of this process is mainly determined by the dif- However, low electron mobility (l ) through meso- 2 -1 -1 fusion coefﬁcient of electrons (D ) and the electron life- porous TiO (*0.1 cm V s ) is a crucial issue and e 2 time (s ). Hence, the nanostructured MOSs with particle imposes severe limitations in enhancing the g of DSSCs size greater than their exciton Bohr radius signiﬁcantly closer to the theoretical limits [117]. One of the main inﬂuence the photoconversion efﬁciency of DSSCs. The hurdles due to inferior l is the electron recombination factors that affect the DSSCs performance are (i) the with the electrolyte if the photoanode layer thickness is mesoporous nature and high surface area of the MOS larger than the diffusion length, the transit length above photoanodes, which allows large amount of dye anchoring which electrons are lost via recombination. resulting in the enhancement of the absorption cross Tin oxide (SnO ) nanostructure, on the other hand, is a section, and (ii) larger amount of density of states (DOS) well-known transparent conducting oxide for nano-elec- 2 -1 -1 in MOS than the molecular orbital of dye enables tronics due to high l (10–125 cm V s ) and wider band speedier injection of electrons from the dye molecule to gap (*3.6 eV) [118–121]. However, its conduction band the MOS. Considering the scenario, inefﬁcient charge minimum occurs at an energy lower than that of TiO [72] transport in the nanostructured MOSs originate from the and, therefore, DSSCs with SnO electrode usually give trapping and detrapping of electrons at the surface atomic low open circuit voltages (V B 600 mV) [122]. OC states in the electronic band. Moreover, the nanostruc- Recently, V up to *600 mV has been achieved by OC tured MOS photoanodes consist of large amount of sur- preparing the SnO core/shell electrodes and/or making face atoms resulting in greater degree of trap density. The composite electrode with other wide-band-gap semicon- trapping and detrapping process lowers the kinetic energy ductors [123–125]. To increase the PCE of SnO of the mobile electrons, eventually leading to poorer cell based DSSCs, several approaches have been consid- performance. Quantiﬁcation of the traps and their subse- ered which includes: (i) modifying the electrode surface quent elimination could improve the photovoltaic [126], (ii) modifying electrolyte composition [126, 127], 123 Mater Renew Sustain Energy (2015) 4:11 Page 13 of 25 11 an electrospinning technique as shown in Fig. 13. The ﬂowers also exhibited an enhanced Fermi energy resulting in higher electron mobility [130]. Furthermore, DSSCs fabricated using the SnO ﬂowers resulted in V 2 OC *700 mV and one of the highest photoelectric conversion efﬁciencies achieved using pure SnO . The study also demonstrated that the ﬂowers are characterized by higher chemical capacitance, higher recombination resistance, and lower transport resistance compared with ﬁbers. The effective electron diffusion coefﬁcient and electron mobility in the ﬂowers were an order of magnitude higher than that for the ﬁbers (Fig. 14). One of the most critical challenges in DSSCS research is the rapid recombination rate between the electrons in the conduction band of TiO and the oxidized redox mediator of the electrolytes. Advances in solid-state DSSCs with spiro-OMeTAD as HTM have shown that its usage is limited by the thickness of the photoanodes (*3 mm) due to incomplete percolation [131]. Therefore, pore ﬁlling has become an area of intense research to improve the hole injection dynamics and reduce the recombination rate with Fig. 12 Schematic of the functional principle of a dye-sensitized thickness [132, 133]. Several approaches have been solar cell. E and E are the position of the valence and conduction VB C investigated to improve the charge collection in liquid- and bands of TiO , respectively. The open circuit voltage V is deﬁned 2 OC solid-state electrolytes by using one-dimensional ZnO and by the difference between the Fermi-level E and the redox potential * ? E of the iodide/iodine couple. D /D are the ground state and D / F,redox TiO nanostructures as photoanodes [134–137]. D is the excited state of the sensitizer from which electron injection 3D photoanodes for DSSCs was also developed to into the TiO conduction band occurs overcome such drawbacks. Recently, a novel bottom-up 3D host–passivation–guest (H–P–G) electrode was developed (iii) combining with other MOS nanoparticles [125], and which enabled complete structural control of the nanos- (iv) by using a core–shell conﬁguration of suitable energy tructure that favors efﬁcient electron extraction and band-matched MOS [125, 128, 129]. recombination dynamics with enhanced optical scattering Flower-shaped nanostructures of an archetypical trans- properties for improved light harvesting. This 3D nanos- parent conducting oxide, SnO , have been synthesized by tructure when employed as photoanode in DSSCs Fig. 13 SEM images of SnO photoanodes: a ﬁbers and b ﬂowers [130]. Reproduced by permission of The Royal Society of Chemistry 123 11 Page 14 of 25 Mater Renew Sustain Energy (2015) 4:11 Fig. 14 Schematic illustration of the fabrication method for a 3D host–passivation–guest dye- sensitized solar cell. Reprinted (adapted) with permission from ref [140]. Copyright (2011) American Chemical Society signiﬁcantly improved photocurrent, ﬁll factor, and most chain reaction involved in the photocatalytic oxidation importantly the photovoltage of the device. [114, 138– processes. This technology is preferably used in photocat- 140]. DSSCs employing novel porphyrin sensitizers with alytic dye degradation owing to advantages such as (1) low cobalt (II/III)-based redox electrolyte exhibit high PCE or no toxins, (2) being cheaper, (3) exhibiting tunable [12 % as shown in Table 2 [141]. The speciﬁc molecular physiochemical properties by modifying the nanoparticle design of porphyrin sensitizers signiﬁcantly retards the rate size and doping concentration and (4) good photocatalytic of interfacial back-electron transfer from the conduction lifetime without undergoing substantial loss over a period band of the nanocrystalline titanium dioxide photoanode to of time. [145]. Generally, metal oxide photocatalysis is the oxidized cobalt mediator, leading to the attainment of carried out via advanced oxidation process (AOP) which is extraordinarily high photovoltage of about 1 volt [141]. performed by employing a strong oxidizing species of OH Other similar DSSCs employing various dyes and redox radicals usually produced in situ. The OH radicals form the shuttle mediators are listed in Table 2. trigger to initiate a sequence of reactions that crumbles the complex dye macromolecule into simpler and smaller, less harmful components [10, 146, 147]. Photocatalysis Photocatalysis is the key process that enables the conver- Basic concept of photocatalysis sion of solar energy into chemical energy needed for the decomposition of dyes or organic pollutants. The photo- Photocatalytic reactions are basically a multi-step process catalytic reactions usually occur on the surface of the involving oxidation and reduction reactions as illustrated in semiconductors. Considering the scenario, metal oxide Fig. 15. The photocatalytic processes comprise three fun- semiconductor (MOS) photocatalysts are employed as damental reaction pathways: (1) Photons are absorbed by activators that assist in catalyzing the complex radical the photocatalysts upon illumination from the light source. Table 2 List of high-efﬁciency DSSCs employing MOS photoanodes Photoanodes Dye Redox couple J (mA/cm)V (V) FF (%) PCE (%) Reference SC OC TiO CYC-B11 I =I 20.1 0.74 77 11.5 [141] TiO YD2-o-CB Co(bby) 17.7 0.93 74 12.3 [142] 2 3 TiO Y123 Spiro-OMeTAD 9.5 0.98 77 7.1 [143] SnO N3 I =I 7.3 0.7 60 3.0 [130] ZnO N719 I =I 15.2 0.69 50 5.2 [144] 123 Mater Renew Sustain Energy (2015) 4:11 Page 15 of 25 11 Fig. 15 Schematic diagram explaining the principle of photocatalysis. Reproduced from Ref. [155] with permission from The Royal Society of Chemistry When the photons have higher energies greater than the (i) the band gap or energy separation is sufﬁcient or larger band gap of the photocatalyst material, the electrons from than the energy required for the desired reaction; (ii) the the valence band (VB) are excited to the conduction band redox potentials of the electron and hole corresponding to (CB) forming an electron–hole pair. (2) The photogener- their valence and conduction band are suitable for inducing ated carriers (electrons and holes) have the tendency to redox processes; (iii) the reaction rates of the redox pro- recombine on the surface/bulk of the semiconductor. On cesses are faster than the electron–hole pair recombination the other hand, the electron–hole pairs may also get sepa- rate [151]. rated in a surface space charge region. If the diffusion of Moreover, the additional bottleneck for solar energy the electrons and holes is not hindered by any trap states or conversion by photocatalysis is that most metal oxide- defect states, then the charge carriers would eventually based photocatalyst materials are wide-band-gap semi- reach the surface to trigger chemical reactions by charge conductors. Wide-band-gap semiconductors possess elec- transfer from the photocatalyst to an adsorbate. (3) Finally, tronic band gaps around or larger than 3 eV and therefore the reduction reaction occurs when the photogenerated their performance is conﬁned to the small UV region of the electrons interacts with the absorbed molecules on the solar spectrum. Therefore, the quest for ﬁnding efﬁcient semiconductor (photocatalysts) surface. To facilitate the photocatalysts responsive to visible light took the limelight. electron transfer from the photocatalyst to the adsorbate, Approaches such as doping or development of new mate- the conduction band (CB) minimum of the photocatalyst rials suitable such as oxynitrides took the center stage must be higher than the reduction potential of the adsor- [152–154]. One major drawback of employing dopants is bate. Similarly, the photogenerated holes could generate that they often act as trap states serving as recombination strong oxidizing agents like OH radicals by interacting centers and eventually resulting in performance degrada- with the adsorbed molecules on the surface. Here, the tion over a period of time. On the other hand, dye valence band (VB) maximum of the photocatalyst must be adsorption onto the photocatalyst surface is normally lower than the oxidation potential of the adsorbate for considered the second most essential component for pho- efﬁcient hole transfer [148–150]. tocatalytic dye degradation. One of the major factors that determines the dye adsorption onto the photocatalysts Material and electronic properties required surfaces is the surface area [155]. for photocatalysts From the electronic perspective, efﬁcient diffusion of photoexcited charge carriers to the surface with less The development of efﬁcient photocatalysts for high recombination is a basic requisite of any good photocata- chemical conversion efﬁciency solely relies on the means lyst. This process aids the development of speciﬁc surface to suppress back-electron transfer or electron–hole pair sites to exhibit preferential oxidation or reduction chemical recombination process. Therefore, the electron–hole pair reactions. Several factors determine the efﬁcacy of charge generated can be efﬁciently used for the photocatalytic separation and direction selectivity of charge carriers, purpose, provided they exhibit the following properties: which include (i) band structure, (ii) polarization in case of 123 11 Page 16 of 25 Mater Renew Sustain Energy (2015) 4:11 photocatalysts for efﬁcient and stable long-term performance. Titanium dioxide as photocatalysts TiO is widely employed as a photocatalyst in dye wastewater treatment, mainly due to its capability to gen- erate a highly oxidizing electron–hole pair. Moreover, it has good chemical stability, non-toxicity, and long-term photostability [160–162]. The wide band gap (Eg [ 3.2 eV) of TiO limits its potential, because only high energy light in the UV region with wavelengths \387 nm can instigate the electron–hole separation pro- cess [156, 163]. Therefore, developing a photocatalyst that Fig. 16 Comparison of recombination processes of photogenerated can efﬁciently harness the energy from natural sunlight, carriers within a anatase and b rutile structure. Reproduced from Ref. i.e., from the visible region, is one of the major challenges [170] with permission from the PCCP Owner Societies in this ﬁeld [164]. Numerous modiﬁcations of the structure of TiO have been made to achieve the following: (i) de- ferroelectric materials, and (iii) electrostatic potentials in crease the band gap energy to harness the photons from the the surfaces as a function of charged adsorbate present on visible region; (ii) increase the efﬁciency of electron–hole them [156–158]. Directed charge transport can also be production; and (iii) augment the absorbency of organic facilitated by the electronic band bending present in the pollutants onto TiO by appropriate surface modiﬁcations surface or near surface region of the photocatalysts. Upon [148, 149, 165]. Doping is one of the means to achieve the photoexcitation, the band bending provides sufﬁcient space above-mentioned characteristics. Metal ions of noble charge region, assisting the charge separation of photo- metals (Pt, Pd, Ag, and Au) [166] and transition metals (Cr, generated carriers and also aiding the directional diffusion Cu, Mn, Zn, Co, Fe, and Ni) [167] are used as dopants of the electrons to the surface; which in turn enhances the [168]. Even non-metals such as C, N, S, and P are used for photocatalytic activity of the material. The extent of the this purpose [160, 161, 163, 169]. Transition metals are space charge region is determined by the amount of doping used as an alternative to noble metals to reduce the cost. and the dielectric constant of the material. Furthermore, the Fe-doped TiO has been found to exhibit dye degradation size of the catalyst particle also plays a major role in efﬁciency of about 90 % [163]. determining the band bending at the surface. The catalyst particle should be larger than the space charge layer, The difference in photocatalytic activity otherwise there would be no signiﬁcant potential drop between anatase and rutile TiO toward the surface. The width of the space charge layer L sc depends on the materials properties and surface potential In general, anatase TiO usually exhibits higher photocat- Vs, which in turn depends on the surface charges [159]: alytic activities than rutile TiO [170, 171]. The perfor- 1=2 mance improvement arises from the fundamental 2eV L ¼ L ; SC D difference in the electronic properties between them. kT Anatase TiO is an indirect band gap semiconductor, where L is the Debye length, K is Boltzmann constant and whereas rutile belongs to the direct band gap semicon- T is the temperature. ductor. Anatase structure exhibits longer lifetime of pho- On the other hand, materials with high electric constant togenerated carriers (holes and electrons) than direct band increase the width of the space charge region and hence it gap rutile structure. The reason is that in anatase TiO , the has a direct inﬂuence on the amount of photogenerated direct transitions of photogenerated electrons from the carriers extracted to the surface by means of the band conduction band (CB) to valence band (VB) is not possible. bending or surface potential. It is noteworthy to mention Furthermore, anatase exhibits lower average effective mass that the band bending supports either holes or electrons of photogenerated electrons and holes when compared to transport to the surface depending on the direction of the rutile or brookite structure. The lower effective mass potential, thereby the opposite charge carrier is trapped in enables the rapid transport of photogenerated carriers from the bulk of the material decreasing the photoactivity of the interior to the surface of anatase TiO , thereby resulting the catalysts. Hence, it is very much and equally impor- in lower recombination rate and eventually leading to tant to remove the oppositely charged carriers from the higher photocatalytic activity than rutile structure. 123 Mater Renew Sustain Energy (2015) 4:11 Page 17 of 25 11 Table 3 List of highly efﬁcient ZnO- and TiO -based photocatalysts Catalyst Catalyst Reaction conditions Decolorisation Inference/remarks Reference loading efﬁciency -1 P-TiO 0.25 g L Initial concentration of methylene blue 90 % Photocatalytic activity is [180] -5 -1 (MBu): 1.2 9 10 mol L affected by calcination temperature Irradiation time: 40 min UV irradiation 6 W medium-pressure Hg lamp Optimum calcination k = 254 nm temperature is 700 C P:Ti atomic ratio = 0.01 Improved anatase crystallinity -1 P-Doped 0.2 g L Initial conc. rhodamine B (RhB): 12 ppm 90.3 % under natural Un-doped TiO showed rutile [162] TiO sunlight 98.9 % under phase at 800 C Irradiation time: 40 min UV irradiation Visible irradiation: sunlight UV irradiation: 500 W Hg lamp Stirring 3 h for absorption/desorption equilibrium -1 N-TiO 0.2 g L Initial conc. monoazo (ReactiveRead) 100 % ReR 77 % ReBI No change of anatase to rutile [165] ReR, diazo (Reactive Black) ReBI and ratio on N-doping. Remained 100 % DGr poliazodye (Direct Green) DGr :5 ppm constant at 90:10 Irradiation time: 300 min Adsorption capacity is higher due to nitrogen doping Radiation intensity of about 385 W/m for visible light and 0.09 W/m for UV -1 WOx–TiO 1.0 g L Initial conc. Acid Orange 7 (AO ) and 100 % Dye absorbed on WOx–TiO [181] 2 7 2 methyl orange (MeO): 25 ppm surface decolorized and aromatic ? aliphatic acid Irradiation time: 240 min for AO intermediates formed 300 min for MeO visible irradiation: 1000 W halogen lamp short-wavelength components (\420 nm) of the light were cut off using a cutoff glass ﬁlter -1 TiO 0.5 g L Initial conc. MeO: 2 ppm 77.19 % Anatase nanocrystals showed [182] good photocatalytic activity Irradiation time: 540 min in the degradation of UV irradiation: two 6 W ultraviolet light methylorange bulbs (light bulb k = 360 nm) -1 ZnO 0.5 g L Initial conc. Basic Blue 11 (BB-11): 100 % Hydroxyl radical formation [183] 50 ppm better at higher pH Irradiation time: 24 h Visible irradiation: 2 9 15 W visible lamps -1 ZnO 1.25 g L Initial conc. RemBBu(R) RemBl(B): 100 % RemBBu(R), Oxygen important to scavenge [184] 50 ppm RemBl(B): total photo-generated electrons organic carbon (TOC) and prevent electron–hole Irradiation time: 60 min removal: 80 % pair recombination UV irradiation: 125 W Philips Hg lamp RemBl(B); 90 % Toxicity tests showed ZnO not k [ 254 nm pH: 5.0 RemBBu(R) efﬁcient for toxic removal of RemBBu(R) Prolonged degradation increased toxicity -1 ZnO vs. 2.0 g L Initial conc. RemR(F-3B): 150 ppm 100 % TiO /254 nm Comparison of efﬁciency of [185] TiO UV TiO and ZnO photocatalysts 2 2 Irradiation time: 60 min in various parameters: pH, *90 % TiO /365 nm UV irradiation: 6 9 6 W UV lamps light intensity, initial UV k = 365 and 254 nm, intensity 1.85 and concentration, catalyst -2 *95 % ZnO/254 nm 1.65 mWcm loading UV 100 % ZnO/365 nm UV 123 11 Page 18 of 25 Mater Renew Sustain Energy (2015) 4:11 Table 3 continued Catalyst Catalyst Reaction conditions Decolorisation Inference/remarks Reference loading efﬁciency -1 ZnO-TiO 0.5 g L Initial conc. CoR: 5 ppm 100 % Photocatalytically inactive [186] composites Zn TiO and larger band gap 2 4 Irradiation time: 10 min of ZnTiO formed at temp. UV irradiation: 30 W UV lamp [680 C during calcination under O atmosphere calcination 2 ZnO with TiO enhanced temp.: 420 C photocatalytic degradation Adapted from Reference [10] Fig. 17 a Schematic showing the surface plasmon decay processes in overcome the Schottky barrier (/ / - v are injected into SB = M s) which the localized surface plasmons undergoes radiative decay via the conduction band of the adjacent semiconductor, where / is the re-emitted photons (left) or non-radiative decay via excitation of hot work function of the metal and v is the electron afﬁnity of the electrons (right). b Electrons from occupied energy levels are excited semiconductor. Reprinted with permission from Ref. [187]. Copyright above the Fermi energy (Plasmonic energy conversion). c Hot 2014 Nature Publishing Group electrons generated by the plasmons with sufﬁcient energy to Moreover, the electron afﬁnity of anatase is higher than UV and visible light [178]. Other MOSs such as vana- rutile [172]. Therefore, photogenerated conduction elec- dium oxide, tungsten oxide, molybdenum oxide, indium trons will ﬂow from rutile to anatase and this factor is oxide, and cerium oxide have also been studied, but their likely to be the driving force for the increased photoactivity performance is found to be inferior compared to titanium of anatase–rutile composite materials [172]. Comparison of dioxide and zinc oxide [10, 179]. The performance of the recombination processes of photogenerated carriers ZnO- and TiO -based photocatalysts is listed compre- within anatase and rutile structure is shown in Fig. 16. hensively in Table 3. Zinc oxide as a photocatalyst Recent advances in photocatalysts: plasmon-assisted photocatalysis ZnO has a wide band gap (3.2 eV) and the unique elec- trical and optoelectronic property has made it a potential As discussed earlier, the electron–hole separation is of candidate as a photocatalyst. Studies have shown that the paramount importance for realizing higher conversion performance of ZnO under visible light is much more efﬁciencies in photovoltaic and photocatalytic devices. efﬁcient than TiO [173–175]. Though it is highly Employing plasmonic technology for energy conversion is effective under the inﬂuence of UV light [9, 176], with found to be a promising alternative to the conventional suitable physio-chemical modiﬁcations or by doping, ZnO electron–hole separation in semiconductor devices. This can be used as a visible light photocatalyst. Apart from technique involves generation of hot electrons in plasmonic this, the usage of higher-intensity (500 W) visible light is nanostructures by means of electromagnetic decay of sur- found to increase the photocatalytic activity of the ZnO face plasmons [187–190]. The working principle of the nanoparticles [177]. ZnO photocatalyst is also found to be plasmonic energy conversion at the semiconductor inter- better than SnO , CdS and ZnS for dye degradation under face is depicted in Fig. 17. When the metal nanoparticle 123 Mater Renew Sustain Energy (2015) 4:11 Page 19 of 25 11 Fig. 18 a and b TEM images of the Au/TiO nanostructure. c Comparison of decomposition of MO dye by commercial TiO NPs (P25) and Au/ TiO nanostructure prepared by different methods. Reprinted (adapted) with permission from Ref [195]. Copyright (2014) American Chemical Society (plasmon) is illuminated with highly energetic photons, hot activity of the Au/TiO depend on numerous variables. The electrons are generated in conditions of non-radiative intensity of the SPR strongly relies on the shape of the Au decay. Hot electrons whose energies are sufﬁciently higher and TiO NPs. Other factors include (i) dielectric constant than the work function of the material get injected into the of the surrounding medium, (ii) quantum mechani- neighboring semiconductor, thereby producing a pho- cal/electronic interactions between the ligands (stabilizers) tocurrent. This interesting phenomenon caught the atten- and the nanoparticles, and (iii) monodispersity of the NPs tion of the researchers and led to the development of [194]. Au/TiO photocatalysts prepared by depositing pre- NMNPs (noble metal nanoparticles)/semiconductor synthesized colloidal Au nanoparticles onto TiO nanostructures as photocatalysts. Among the various nanocrystals of precisely controlled size and morphology structures thus produced with this combination, Au/TiO through a delicately designed ligand-exchange method nanostructures show promising prospect, as the surface resulted in Au/TiO Schottky contact with low energetic plasmon resonance (SPR) effect in these structures charge transfer barrier. The photocatalysts thus obtained by enhances the photoactivity of titania under visible light. In this strategy showed superior activity compared to con- this process, the photogenerated electrons possess negative ventionally prepared photocatalysts in dye decomposition potential higher than that of the conduction band of the under visible-light illumination [195]. Figure 18 shows that TiO ; thereby, the photogenerated electrons transfer efﬁ- the rate of decomposition of the dye is signiﬁcantly higher ciently from excited Au NPs to TiO NPs [189–191]. For than that of the commercial/pure TiO NPs. Thus, the mode 2 2 instance, in the event of degradation of the pollutant of deposition of the Au and TiO NPs signiﬁcantly affects 4-chlorophenol (4CP) using P-25 titania (commercial the performance of the photocatalysts. Therefore, under- TiO ), the Au NPs signiﬁcantly enhanced the catalytic standing the complex processes affecting the photocatalytic activity by 80 % [192]. The photocatalytic mineralization performance is indispensable for the rational design of the of 4-CP exhibited by Au/TiO is higher than Pt/TiO [ ideal noble metal-modiﬁed metal oxide semiconductor 2 2 Ag/TiO [ TiO [193]. Factors affecting the photocatalytic photocatalysts. 2 2 123 11 Page 20 of 25 Mater Renew Sustain Energy (2015) 4:11 their evolution (2012–2050). J Renew Sustain Energy 5, 023112 Conclusion (2013) 6. Razykov, T.M., Ferekides, C.S., Morel, D., Stefanakos, E., In the future, the photovoltaic market depends not only on Ullal, H.S., Upadhyaya, H.M.: Solar photovoltaic electricity: our ability to increase power conversion efﬁciencies, but current status and future prospects. Sol. Energy 85, 1580–1608 (2011) also on the stability of the devices. Moreover, the pho- 7. Norris, D.J., Aydil, E.S.: Materials science. Getting Moore from toanodes in DSSCs require high sintering temperature, solar cells. 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Metal oxide semiconducting interfacial layers for photovoltaic and photocatalytic applications

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